Among the Romans two dimensions were in general use, as may be inferred from the frequent mention in inscriptions or elsewhere of the _sesquipedales_ and of _bipedales_,[2403] or two-foot bricks, as we shall have occasion to show later. Being very flat and thin in proportion to their size, these bricks rather resemble tiles, as has been already noted; they are generally square, or at least rectangular. But there were also _tegulae bessales_ or bricks measuring two-thirds of a foot square, _i.e._ about 8 inches, and triangular bricks, equilateral in form, with a length varying from 4 to 14 inches. The latter are the kind used in all existing Roman walls of concrete with brick facings. The thickness varies from 1¼ to 2 inches. They are not always made with mechanical accuracy, the edges being rounded and the sides not always parallel. In military works they were often used alternately with flint and stone (see below, p. 337), as we see them in England, at Colchester, Dover, Verulam, and many other places.[2404] At Verulam the tiles are arranged in three horizontal layers at intervals of about 4 feet, with flint and mortar between. They were also used for turning the arches of doorways, and for this purpose _tegulae bipedales_ were cut into pieces, so as only to tail a few inches into the concrete which they cover. Complete squares were introduced at intervals to improve the bonding.[2405] The pillars of the floors of hypocausts were formed of _tegulae bessales_, and sometimes also of two semicircular bricks joined so as to form a circle, varying from 6 to 15 inches in diameter.[2406] Occasionally the upper bricks diminished in size, in order to give greater solidity to the structure. The bricks or tiles forming the upper floors were from 18 to 20 inches square; in some cases, as at Cirencester,[2407] these were flanged tiles (see below). The general size of Roman bricks was, in the case of the _sesquipedales_, 1½ by 1 Roman foot; but variations are found, such as 15 by 14 inches. For the _bipedales_ Palladius recommends 2 feet by 1 foot by 4 inches. The great building at Trier known as the Palace of Constantine is built of burnt bricks, 15 inches square by 1¼ inch thick.[2408] Prof. Middleton notes tiles in Rome of 12, 14, and 18 inches square,[2409] and Marquardt[2410] states that bricks found in France measure 15 by 8 to 10 inches; others (the _bessales_) 8 by 8 by 3 inches. A complete circular brick, measuring 7½ inches across by 3¼ inches thick, and impressed with the stamp of the eleventh legion, was found at Dolae near Gardun, and is now in the museum at Spalato.[2411]