The form of arched vault that had served as the lowly dwelling of a primitive people was retained in its entirety for the roof of later and larger buildings; the walls, whether of logs or shaped timber, served as imposts, just as the soil had done, and so the bulbous domes, the square and oblong attic roofs with their characteristic gonflements have been retained. It is merely an example of the persistence as decoration of forms which were originally wholly utilitarian. This is particularly the case with the double arch where the pendant keystone descends to the level of the imposts and is of course supported from the lintel when executed in masonry. Another characteristic Russian form is the circular arch of masonry, which has the voussoirs of the intrados of the usual regular form but of the extrados slightly elevated at the corner to indicate the "ogival arch," which was the common form of the wooden arch in Moscow. As already stated (ch. ii.) the early forms of Russian dwellings may be studied from the models in the Historical Museum; one peculiarity is that each successive storey is set back from that immediately below instead of projecting as in the half-timbered houses, of mediæval England. In addition to the belvederes of the Terem and Potieshni Dvorets, it is noticeable in the towers of the Kremlin wall. They were originally of timber and the earlier form is retained--even to the double walls and tiers--so necessary to a wooden bulwark, but quite foreign to the method of the Italian masons who erected these buildings. The steep roofs of the towers is also common and convenient in constructing with timber, but needless and difficult when working with tiles and bricks. So long as these remain the wooden original Moscow cannot be wholly forgotten. The attempt to retain the pyramidal or retreating form when building with bricks has resulted in a distinctly Muscovite style for towers and spires. Instead of a parapet on the walls of the tower, a tier of small circular arches is imposed, and form the crowns of these, also set back, spring the voussoirs of a second tier, and in like manner other tiers until the desired height is reached for the spire, or the cylindrical shaft that is to support the dome, or whatever other ornament is used to crown the structure. One of the best examples of this form is the church of the Nativity on the Mala Dmitrovka, which was built in the "golden" period of Moscow--1625-1680--when for all buildings of first importance masonry had supplanted the use of wood (see p. 181). The earlier form may be seen in the roof of the Blagovieshchenski Sobor; and the varieties of pattern are reproduced in the attic roofs of the Historical Museum building. The absurdity of the pendant keystone in the double arch is demonstrated by the arch over the doorway to the courtyard of the synod, and the lintels of doors and windows of the Potieshni Dvorets.