The Romans confined their domes to circular or polygonal buildings. Meanwhile they had worked out the construction of groined vaulting upon four supports. The Byzantine achievement was to make four supports carry a dome. It was accomplished by developing the element of construction--the _pendentive_. We have already noted the bas-relief found at Koyunjik, which shows that the Assyrians understood the crowning of small square buildings with domes. While actual examples have perished, the tradition of this construction seems to have survived in the East. For in the third century <small>A.D.</small>, when the Persians established the Sassanian Empire under the impulse of a movement that sought to restore the ideals and habits of the old national life, the builders erected domes in the palaces of Serbistan and Firuzabad. The method they adopted was to bridge each angle of the square, at some distance below the top, with a small arch. On these they erected two small arches that projected beyond the face of the original arch and accordingly extended the width of the bridge. They continued this process of superimposing tier upon tier of arches, until the bridge was level with the top of the square, by which time the latter was transformed into an octagon. Then, by inserting a _corbel_ or bracket in each angle of the octagon and taking advantage of the thickness of the masonry, they were able to adjust a dome to the structure. This system of dome-support, we shall find, was adopted in Gothic architecture, where the arches are called _squinches_. Another method of dome-support, found in the =Mosque of Damascus= and frequently employed in the churches of Asia Minor, was to bridge the angle with a semi-circular niche. Meanwhile what the Byzantine architects developed was a geometrically exact system of converting the square into a circle by means of concave triangular members that are specifically called _pendentives_.