In the prayer hall, the 414 columns of marble, granite or porphyry (among more than 500 columns in the whole mosque), taken from ancient sites in the country such as Sbeitla, Carthage, Hadrumetum and Chemtou, support the horseshoe arches. A legend says they could not count them without going blind. The capitals resting on the column shafts offer a wide variety of shapes and styles (Corinthian, Ionic, Composite, etc.). Some capitals were carved for the mosque, but others come from Roman or Byzantine buildings (dating from the second to sixth century) and were reused. According to the German archaeologist Christian Ewert, the special arrangement of reused columns and capitals surrounding the mihrab obeys to a well-defined program and would draw symbolically the plan of the Dome of the Rock. The shafts of the columns are carved in marble of different colors and different backgrounds. Those in white marble come from Italy, some shafts located in the area of the mihrab are in red porphyry imported from Egypt, while those made of greenish or pink marble are from quarries of Chemtou, in the north-west of current Tunisia. Although the shafts are of varying heights, the columns are ingeniously arranged to support fallen arches harmoniously. The height difference is compensated by the development of variable bases, capitals and crossbeams; a number of these crossbeams are in cedar wood. The wooden rods, which usually sink to the base of the transom, connect the columns together and maintain the spacing of the arches, thus enhancing the stability of all structures which support the ceiling of the prayer hall.