At the beginning of the fifth century, which is a suitable point from which to date the rise of Byzantine architecture, three principal types of church plan prevailed in the Roman world:—

I. The Basilica: an oblong hall divided into nave and aisles, and roofed in wood, as in the Italian and Salonican examples, or with stone barrel-vaults, as in Asia Minor and Central Syria.

II. The Octagonal or Circular plan covered with a stone or brick dome, a type which may be subdivided according as (1) the dome rests upon the outer walls of the building, or (2) on columns or piers surrounded by an ambulatory.

The Pantheon and the so-called Temple of Minerva Medica at Rome are early examples of the first variety, the first circular, the second a decagon in plan. S. George at Salonica is a later circular example. An early instance of the second variety is found in S. Constanza at Rome, and a considerable number of similar churches occur in Asia Minor, dating from the time of Constantine the Great or a little later.

III. The Cross plan. Here we have a square central area covered by a dome, from which extend four vaulted arms constituting a cross. This type also assumes two distinct forms:

(1) Buildings in which the ground plan is cruciform, so that the cross shows externally at the ground level. Churches of this class are usually small, and were probably sepulchral chapels rather than churches for public worship. A good example is the tomb of Galla Placidia at Ravenna.

(2) In the second form of the Cross church the cross is enclosed within a square, and appears only above the roofs of the angle chambers. An example is seen in the late Roman tomb at Kusr en Nûeijîs in Eastern Palestine. In this instance the central square area is covered with a dome on continuous pendentives; the four arms have barrel-vaults, and the angles of the cross are occupied by small chambers, which bring the ground-plan to the square. The building is assigned to the second century, and shows that true though continuous pendentives were known at an early date10 (Fig. 8).

Another example is the Praetorium at Musmiyeh, in Syria, 11 which probably dates from between 160 and 169 A.D. At some later time it was altered to a church, and by a curious foreshadowing of the late Byzantine plan the walls of the internal cross have entirely disappeared from the ground-plan. The dome rests on four columns placed at the inner angles of the cross, and the vaulted cross arms rest on lintels spanning the space between the columns and the outer walls.

From these three types of building are derived the various schemes on which the churches of the Byzantine Empire were planned.

Of the basilican form the only example in Constantinople that retains its original plan is S. John the Baptist of the Studion (p. 56), erected c. 463 A.D.

The church of SS. Sergius and Bacchus (p. 70) and the baptistery of S. Sophia (p. 78) represent respectively the two varieties of the octagonal plan. In the former the dome rests on piers surrounded by an ambulatory; in the latter the dome rests upon the outer walls of the buildings. Both are foundations of Justinian the Great.

Of the Cross church plan showing the cross externally at the ground level no example survives in the city. But at least one church of that form was seen at Constantinople in the case of the church of the Holy Apostles. This was essentially a mausoleum, built originally by Constantine the Great and reconstructed by Justinian to contain the sarcophagi of the sovereigns and the patriarchs of New Rome.12

The church of S. Mark at Venice was built on the plan of the Holy Apostles. It is a cruciform church with aisles, but the galleries which might have been expected above them are omitted. The central dome rests on four piers, and four smaller domes cover the arms.

Professor Strzygowski gives examples of cross-planned cells in the catacombs of Palmyra,13 and in many Eastern rock tombs.14 Such cross plans are found also in the Roman catacombs. These subterranean chapels, of course, do not show the external treatment, yet there can be little doubt that the external cross plan was originally sepulchral, and owes its peculiar system of planning to that fact. On the other hand, it was adopted in such churches as S. Mark's at Venice and in the French examples of Périgord for aesthetic or traditional reasons.

In passing now to a consideration of the distinct forms developed from these pre-Byzantine types of church building, the classification adopted by Professor Strzygowski may be followed. In his Kleinasien he has brought forward a series of buildings which show the manner in which a dome was fitted to the oblong basilica, producing the domed basilica (Küppelbasilica), an evolution which he regards as Hellenistic and Eastern. In contrast to this, Strzygowski distinguishes the domed cross church (Kreutzküppelkirche), of which S. Theodosia in Constantinople (p. 170) is the typical example and which is a Western development. A comparison of the two forms is of great importance for the study of certain Constantinople churches.

The domed basilica, as the name indicates, is a basilica with nave and aisles, in which a square bay in the centre of the nave is covered by a dome on pendentives. To north and south, within the arches supporting the dome, appear the nave and gallery arcades of the basilica; and as the galleried basilica is a usual Eastern form galleries are usual in the domed basilica. As seen from the central area, therefore, the north and south dome arches are filled in with arcades in two stories, and the side aisles and galleries are covered with barrel vaults running parallel to the axis of the church. At the west end a gallery over the narthex may unite the two side galleries. At Kasr ibn Wardan, instanced by Strzygowski as a typical domed basilica,15 there is such a western gallery (Fig. 1). According to Strzygowski the domed basilica is older than the fifth century.

The domed basilica remains always an oblong building, and whilst the two sides to north and south are symmetrical, the western end retains the basilican characteristics—it has no gallery or arcade communicating with the central area. The narthex communicates with the nave by doors, and if a gallery is placed above it, both narthex and gallery are covered by barrel vaults.

In the domed cross church (Kreutzküppelkirche) the central dome rests on barrel vaults which extend to the outer walls of the building and form the arms of the cross, the eastern arm forming the bema. The lighting of the church is by windows in the gable walls which terminate the north, south, and west cross arms. The prothesis and diaconicon open off the side arms, and two small chambers in the western angles of the cross bring the plan externally to the usual rectangular form.

The domed cross church may have galleries, as in S. Theodosia (p. 170), or may be without them, as in SS. Peter and Mark (p. 193). Where galleries are present they are placed in the cross arms and are supported by arcades at the ground level. The vaults beneath the galleries are cross-groined. The domed cross church is a centrally planned church, in contrast to the domed basilica, which is oblong, and therefore we should expect that where galleries are used they will be formed in all three arms of the cross, as is the case in S. Theodosia.

There are a number of churches which vary from these types, but which can generally be placed in one class or the other by the consideration of two main characteristics: if the dome arches extend to the outer walls the building is a domed cross church; if the galleries are screened off from the central area by arcades the building is a domed basilica.

The church at Derè Aghsy,16 for instance, if we had only the plan to guide us, would appear to be a typical domed basilica (Fig. 2), but on examining the section we find that the north and south dome arches extend over the galleries to the outer walls and form cross arms (Fig. 3). The building is, in fact, a domed cross church with no gallery in the western arm. Above the narthex at the west end, and separated from the western cross arm, is a gallery of the type usual in the domed basilica, so that Derè Aghsy may be regarded as a domed cross church with features derived from the domed basilica. S. Sophia at Constantinople, the highest development of the domed basilica, has a very similar western gallery.

The church of S. Nicholas at Myra 17 (Fig. 4) has a gallery at the west end, but the cross arms do not appear to be carried over the galleries. The plan is oblong and the cross-groined vault is not used. The church, therefore, takes its place as a domed basilica.

The church of the Koimesis at Nicaea 18 (Figs. 5 and 6) has no galleries to the sides. The aisles open into the central area by arcades, above which are triple windows over the aisle vaults. At the western end is a gallery above the narthex. The aisles are barrel-vaulted, and as the church is planned on an axis from east to west, and is not symmetrical on all three sides, it is regarded as a domed basilica. It is such a form as might be developed from a basilica without galleries.

In Constantinople there are three churches which seem to constitute a type apart, though resembling in many ways the types just considered. They are S. Andrew in Krisei, (p. 117), S. Mary Pammakaristos (p. 150), and S. Mary Panachrantos (p. 130). In these churches, as originally built, the central dome is carried on four arches which rise above a one-storied aisle or ambulatory, allowing of windows in the dome arches on three sides—the eastern dome arch being prolonged to form the bema. The dome arches have arcades communicating with the ambulatory on the north, south, and west. The vaulting is executed either with barrel or with cross-groined vaults. These churches are evidently planned from a centre, not, like the domed basilicas, from a longitudinal axis. At the same time the absence of any cross arms differentiates them from the domed cross churches. S. Andrew, which still retains its western arcade, dates from at least the sixth century, so that the type was in use during the great period of Byzantine architecture. Indeed, we should be inclined to regard S. Andrew as a square form of SS. Sergius and Bacchus, but without galleries. The type is a natural development from the octagonal domed church with its surrounding ambulatory.

The typical late Byzantine church is a development from the domed cross plan. In three examples in Constantinople, S. Theodosia (pp. 170, 172), S. Mary Diaconissa (p. 185), and SS. Peter and Mark (p. 193), we can trace the gradual disappearance of the galleries. S. Theodosia, as has already been mentioned, has galleries in all three cross arms. In S. Mary Diaconissa they are confined to the four angles between the cross arms; SS. Peter and Mark is a simple cross plan without galleries. In later times it became customary to build many small churches, with the result that the chambers at the angles of the cross, of little account even in a large church, were now too diminutive to be of any value, and the question how to provide as much room as possible for the worshippers became paramount. Accordingly the dome piers were reduced to mere columns connected with the outer walls of the building by arches; and thus was produced the typical late Byzantine plan—at the ground level a square, enclosing four columns; above, a Greek cross with a dome on the centre.

From its distinguishing feature this type has been styled the 'four column' plan. It appears in many Constantinopolitan churches, as, for example, S. Theodore (p. 248) and S. Saviour Pantepoptes (p. 214). The cross arms are not always equal, and may be covered with barrel vaults (p. 214 or with cross-groined vaults (p. 198). The bema is usually a bay added to the eastern arm. The angle chambers have either cross-groined vaults or flat dome vaults. In general the churches of this type in Constantinople do not differ from the numerous churches of the same class in the provinces.19

A lobed cruciform plan is found in only one church in Constantinople, that of S. Mary of the Mongols (p. 277). Here the central dome is supported on four piers set across the angles of the square, so that the pendentives do not come to a point as usual, but spring from the face of the piers. Against each side of the square a semidome is set, thus producing a quatrefoil plan at the vaulting level.

Both trefoiled and quatrefoiled churches are not uncommon in Armenia, such as the cathedral at Etschmiadzin; 20 trefoiled churches of a later date are found in the western provinces, and examples have been published from Servia,21 Salonica, 22 and Greece. 23

An unusual form of the cross plan is seen in the building known as Sanjakdar Mesjedi (p. 267), where a cross is placed within an octagon. Probably the building was not originally a church. It resembles the octagon near the Pantokrator (p. 270), and may, like it, have been a library.

Single Hall Churches.—The plans hitherto considered have all been characterised by the presence of aisles, galleries, or other spaces adjoining the central area. The churches of the present class consist simply of an oblong hall, terminating in an apse, and either roofed in wood, or covered with domes placed longitudinally, and resting to north and south on wall arches. Examples of this plan are found in Monastir Mesjedi (p. 264), S. Thekla (p. 211), Bogdan Serai (p. 284), and in the memorial chapels attached to the Pantokrator (p. 235), and the Chora (p. 309). In the case of these two memorial chapels, their narrow, long-stretched plan is evidently due to the desire to keep their eastern apses in line with the east end of the churches they adjoin, and at the same time to bring the western end to the narthex from which they were entered. They are covered with two domes, a system perhaps derived from S. Irene (p. 94). Kefelé Mesjedi (p. 257), which at first sight resembles a single hall church roofed, in wood, was a refectory. Its plan may be compared with that of the refectory at the monastery of S. Luke at Stiris. 24

Apses.—A fully developed Byzantine church terminated in three apses: a large apse, with the bema or presbytery, in the centre; on the right, the apse of the prothesis where the sacrament was prepared; on the left, the apse of the diaconicon, where the sacred vessels were kept. Although there is proof that the prothesis and the diaconicon were in use at a very early period, yet many churches of the great period, as for example S. John of the Studion, SS. Sergius and Bacchus, and S. Sophia, dispensed with these chambers as distinct parts of the building. They were also omitted in small churches of a late date, where they were replaced by niches on either side of the bema. The three apses usually project from the east wall of the church, but occasionally p. 248) the two lateral apses are sunk in the wall, and only the central apse shows on the exterior. As a rule the apses are circular within and polygonal without. It is rare to find them circular on both the interior and the exterior (p. 203), and in Greece such a feature is generally an indication of late date. An octagonal plan, in which three sides of the octagon appear, sometimes with short returns to the wall, is the most common; but in later churches polygons of more sides are used, especially for the central apse, and these are often very irregularly set out. Some of the churches of Constantinople show five, and even seven sides.

Bema.—The bema is rectangular, and sometimes has concave niches on each side (p. 130). It is covered either with a barrel or with a cross-groined vault, and communicates with the prothesis and the diaconicon.

Prothesis and Diaconicon.—These chambers are either square (p. 214) or have a long limb to the east resembling a miniature bema (p. 214). They are lower than the central apse and the cross arms, so that the cruciform figure of the church shows clearly above them on the exterior,25 though in some churches with galleries small chapels overlooking the bema are placed above them at the gallery level (S. Theodosia). They have usually a niche on three sides, and are either dome vaulted or have cross-groined vaults. The combination of a cross-groined vault with four niches springing from the vaulting level is particularly effective. In S. Saviour in the Chora (p. 307) these chambers are covered with drum domes, pierced with windows, but this treatment is quite exceptional.

The Gynecaeum.—In the development of church building, the gynecaeum, or gallery for women, tends to become less and less important. In S. Sophia, S. Irene, and S. Theodosia, the gallery is a part of the structure. In S. Mary Diaconissa (p. 185) it is reduced to four boxes at the angles of the cross, while in S. Mary Pammakaristos and SS. Peter and Mark it is absent (pp. 149, 193). But though no longer a structural part of the church, a gynecaeum appears over the narthex in the latest type of church (p. 215). It is generally vaulted in three bays, corresponding to the three bays of the narthex below, and opens by three arches into the centre cross arm of the church and into the aisles.

The Narthex.—Unlike the gynecaeum, the narthex tends in later times to become of greater importance, and to add a narthex was a favourite method of increasing the size of a church. In basilican churches, like S. John of the Studion, the narthex was a long hall in three bays annexed to the west side of the building, and formed the east side of the atrium. In domed cross churches with galleries the passage under the western gallery was used as a narthex, being cut off from the central area by the screen arcade which supported the gallery. Such a narthex has been styled a 'structural narthex,' as forming an essential part of the central building. It occurs in several of the churches of the city (p. 114).

In domed cross churches without galleries, and in churches of the 'four column' type, neither narthex nor gallery was possible within the cross, and accordingly the narthex was added to the west end. It is usually in three bays and opens into the aisles and central area. Frequently the ends of the narthex terminate in shallow niches (p. 198). In many churches a second narthex was added (p. 166) to the first, sometimes projecting an additional bay at each end, and communicating with halls or chapels on the north or south, or on both sides of the church (p. 128). S. Mark's at Venice presents a fine example of such an extension of the narthex.

When a church could not be sufficiently enlarged by additional narthexes, a second church was built alongside the first, and both churches were joined by a narthex which extended along the front of the two buildings. S. Mary Panachrantos (p. 128) is a good example of how a church could be thus enlarged from a simple square building into a maze of passages and domes.

The Interior.—The natural division, in height, of an early church, whether basilican or domical, was into three stories—the ground level, the gallery level, and the clearstory or vault level. In the West these structural divisions were developed into the triple composition of nave-arcade, triforium, and clearstory. In the East, in conjunction with the dome, these divisions survive in many examples of the later period. Still, Byzantine architecture was more concerned with spaces than with lines. Large surfaces for marble, painting, or mosaic were of prime importance, and with the disappearance of the gallery the string-course marking the level of the gallery also tended to disappear. In churches with galleries, like S. Theodosia (p. 170) and S. Mary Diaconissa (p. 185), the string-courses fulfil their function, the first marking the gallery level, the second the springing of the vault. In SS. Peter and Mark (p. 193), which has no gallery, there is only one string-course, corresponding in level to the original gallery string-course; accordingly the main arches are highly stilted above it. The absence of the second string-course is a faulty development, for a string-course at the vault level would be a functional member, whereas at the gallery level it is meaningless.

In the Panachrantos (p. 130), as well as in other churches without a gallery, the gallery string-course is omitted by a more logical development, and the string-course at the springing of the vault is retained. Openings which do not cut into the vault are then frankly arched, without impost moulding of any kind. Simple vaulted halls, narthexes, and passages have usually a string-course at the vaulting level, broken round shallow pilasters as at the Chora, S. Theodosia, and the Myrelaion. Sometimes the string-courses or the pilasters or both are omitted, and their places are respectively taken by horizontal and vertical bands. Decorative pilasters flush with the wall are employed in the marble incrustation of S. Sophia.

In churches of the 'four column' type the full triple division is common but with a change in purpose. A gallery in a church of this character is not possible, for the piers between which the gallery was placed have dwindled into single shafts. Hence the first string-course ceases to mark a gallery level and becomes the abacus level of the dome columns, as in the north and in the south churches of the Pantokrator. It is then carried round the building, and forms the impost moulding of the side arches in the bema and of the east window. Sometimes, however, it does not extend round the bema and apse but is confined to the central part of the church, as in the Myrelaion, S. Theodore, and the Pantepoptes. On the other hand, in at least one case, the parecclesion of the Pammakaristos, the central part of the chapel is designed in the usual three tiers, but the apse and bema vaults spring from the lower or abacus string-course, leaving a lunette in the dome arch above pierced by a large window. A corresponding lunette at the west end opens into the gynecaeum of the chapel. In S. John in Trullo the two string-courses coalesce and the arches connecting the columns with the walls cut into the stilted part of the dome arches, with the result that all the structural arches and vaults spring from the same level.

Arches.—Though the pointed arch was known and employed in cisterns, as in the Cistern of the One Thousand and One Columns, Bin-bir-derek, the circular arch is invariably found in work meant to be seen. The difficulty attending this form, in which arches of unequal breadth do not rise to the same height, was overcome, as in the West, by stilting, that is, by raising the smaller arches on straight 'legs' to the required height. The stilted arch, indeed, seems to have been admired for its own sake, as we find it used almost universally both in vaulting and in decorative arches even where it was not structurally required. In windows and in the arches connecting the dome columns to the wall stilting is sometimes carried to extremes.

Domes.—The eastern dome of S. Irene, erected about 740 A.D., is generally considered to be the first example of a dome built on a high drum, though S. Sophia of Salonica, an earlier structure, has a low imperfect drum. After this date the characteristics of the Byzantine dome are the high drum divided by ribs or hollow segments on the interior, polygonal on the exterior, and crowned by a cornice which is arched over the windows.26

Drumless domes are sometimes found in the later churches, as in the narthexes of the Panachrantos and S. Andrew, the angle domes of S. Theodosia, and in Bogdan Serai. These are ribless hemispherical domes of the type shown in Fig. 8, and are in all cases without windows. The earlier system of piercing windows through the dome does not occur in the later churches, though characteristic of Turkish work.

The three diagrams (Figs. 8, 9, and 10) illustrate the development of the dome: firstly, the low saucer dome or dome-vault in which dome and pendentives are part of the same spherical surface; secondly, the hemispherical dome on pendentives; and thirdly, the hemispherical dome with a drum interposed between it and the pendentives.

Flat external cornices on the dome are not uncommon in the later churches of Byzantine Greece, as in S. Sophia at Monemvasia.27 In Constantinople only one dome with a flat cornice can be regarded as original, that of S. John in Trullo, a church which is exceptional also in other respects. The many other domes in the churches of Constantinople on high drums and with flat cornices are Turkish either in whole or in part. The high ribless domes of the Panachrantos, for instance, circular in plan within and without, with square-headed windows, plain stone sill, and flat cornice in moulded plaster, may be regarded as typical Turkish drum-domes. As will appear in the sequel, the dome over the north church of the Pantokrator and the domes of SS. Peter and Mark, the Diaconissa, and S. Theodosia, are also Turkish.

It is most unfortunate that the domes of these three domed cross churches have been altered, especially as the domes of S. Mary Diaconissa and S. Theodosia are larger than any of the later domes except the large oval dome on the central church of the Pantokrator which is almost of the same size. It is therefore now difficult to say what was the precise form of the original domes. Most probably they were polygonal drum-domes, and their collapse owing to their size may well have led to the small drum-domes of later times. Though not strictly Byzantine these Turkish domes are of interest as showing the development of Byzantine forms under Turkish rule, and that reversion to the earlier drumless dome which is so marked a feature of the imperial mosques of the city.

Domes are either eight, twelve, or sixteen sided, and usually have a window in each side. These numbers arise naturally from setting a window at each of the cardinal points and then placing one, two, or three windows between, according to the size of the dome. Internally the compartments are separated by broad, flat ribs, or are concave and form a series of ridges on the dome which die out towards the crown. In sixteen-sided domes of the latter type the alternate sides sometimes correspond to the piers outside, so that the dome which has sixteen sides within shows only eight sides without, as in the narthex of S. Theodore (p. 246). The octagonal dome of the Myrelaion (p. 198) seems to have had only four windows from the beginning.

The ribs of a Byzantine dome are not constructive in the same way as are the ribs of a Gothic vault. They were built along with the rest of the dome and of the same material, and are in no way separate from the infilling, though they no doubt strengthened the shell of the dome by their form28. On the outside a circular shaft with a very simple cap is often placed at the angles of the piers, and from these shafts the brick cornice springs in a series of arches over the windows. Sometimes the angle is formed by a point between two half-shafts, as in the domes of the narthex in S. Theodore (p. 246).

External Treatment.—In the older churches the exterior seems to have been left in simple masses of brickwork, impressive only by their size and proportion. Probably even this effect was not considered of great importance. In later times a very beautiful system of decoration with slender shallow niches was introduced and was applied in particular to the east end and to the apses. The finest examples of this system on a large scale are seen at the Pantokrator (p. 235) and S. Theodosia (p. 173). Carefully considered or elaborate external compositions are rare, and the only examples in Constantinople are the side chapel of the Pammakaristos (p. 154) and the narthex of S. Theodore (p. 246).

External Marble and Mosaic.—Marble and mosaic, we have reason to know, were occasionally used on the exterior of churches,29 though no fragments remain. On the south side of the Pantepoptes (p. 216) the string-course does not correspond to the line of the walls, but projects in a manner which shows that marble must have been employed to line the large windows. A similar projection of the string-course or cornice is not uncommon elsewhere, though not so evident as in the Pantepoptes, and may have been made to receive a marble or mosaic lining.

Doors and Windows.—It is a primary rule in Byzantine architecture that all constructive openings are arched. Whatever may be the eventual form of a door or window the opening is first built in brick with a semicircular head, and into this opening the marble jambs and lining are fitted leaving a semicircular lunette above. Doors are square-headed, with heavily moulded architraves and cornice, and the lintel is mitred into the jambs instead of having the more constructive horizontal joint used in the West.

The doors made of wood or of wood lined with bronze, swing on top and bottom pivots which turned in bronze-lined sockets in lintel and threshold. They closed with a rebate in the jambs and against the raised threshold. Windows were sometimes filled in a similar manner, as in the palace of the Porphyrogenitus and in the north gallery of S. Saviour in the Chora (Fig. 100). In the latter double windows or shutters were employed, opening inwards in the same way as did the doors. These shutters may perhaps be regarded as domestic, for in the churches, as is still seen in S. Sophia though the arrangement has vanished elsewhere, the entire arched opening was usually filled in with a pierced marble grille.

In addition to the simple round-headed windows double and triple windows are found. Double windows were naturally formed by dividing the single arch by a central pier. This method presented two varieties: either the pier was continued up to the containing arch, thus giving two pointed lights, or the two lights were covered by separate arches within the main arch. Both methods are used in the narthex of S. Theodore (p. 247). Another variety was produced by placing two single lights together, with a shaft between them instead of the central pier. But as double windows are not very satisfactory, triple windows are more common. In this case both the methods just described of forming the windows were adopted. A large semicircular opening divided by two piers will give an arched light between two pointed lights, or three arched lights, as in the narthex of S. Theodore. In the former case, if shafts are substituted for the piers, a little adjustment will produce the beautiful form found in the side-chapels of the Pammakaristos (p. 152), and of S. Saviour in the Chora (p. 310), where the two side lights are covered by half-arches whose crowns abut on the capitals of the shafts, while between and above them rises the semicircular head of the central light.

The method of grouping three arched windows of the same height is adopted in apse windows, each of them occupying one side of the exterior. As the deep, narrow mullions are set radiating, the arch is narrower inside than outside. But this difficulty was overcome, partly by lowering the inner crowns, so that the arch is conical, partly by winding the surface. In the Pantokrator (p. 238), instead of radiating to the centre of the apse, the side and mullions are placed parallel to the axis of the church, thus obviating all difficulty. Generally the centre to which the mullions radiate is considerably beyond the apse, so that any necessary little adjustment of the arch could easily be made.

Triple windows supported on circular columns are not infrequent in the north and south cross arms. Sometimes the central light is larger than the lateral lights, at other times, as in the Pantepoptes, the three lights are equal. The lower part of these windows was probably filled in with a breastwork of carved slabs, as in S. Sophia, while the upper part was filled by a pierced grille. At present the existing examples of these windows have been built up to the abaci of the capitals, but in the church of S. Mary Diaconissa (p. 186) the columns still show the original form on the inside.

Vaulting.—All Byzantine churches of any importance are vaulted in brick. The only exception to this rule in Constantinople is the little church known as Monastir Mesjedi (p. 264). The different systems of Byzantine vaulting have been so fully treated by Choisy and other authorities, that in the absence of any large amount of new material it is not necessary to give here more than a few notes on the application of these systems in Constantinople. It should always be kept in view that, as these vaults were constructed with the lightest of centering, the surfaces and curves must have been largely determined by the mason as he built, and would not necessarily follow any definite geometrical development. "Il serait illusoire," remarks Choisy, "d'attribuer à toutes les voutes byzantines un trace géométrique rigoureusement défini." 30