When we turn from the stage to the stage-buildings of this period, our information is very incomplete owing to the scanty nature of the ruins. But it is probable that the stage-buildings began about this period to assume a more imposing appearance than in former times. We have seen that in the case of the earlier buildings there is some doubt whether the upper stories were made of wood or of stone. After the beginning of the third century it is probable that stone began to be used for all stories alike.[425] The buildings must also have been of a considerable height, to allow of a suitable back-scene above the tall proscenium.[426] As to the decoration of the wall at the back of the stage nothing is known. In the later Greek theatres, built in the Roman fashion, this wall was constructed in an elaborate architectural design. It usually consisted of two or three rows of columns, rising one above the other, and each surmounted with appropriate entablatures and pediments. Its height was often as great at the top of the auditorium—an arrangement which was found to improve the acoustic properties of the theatre. Back-walls of this sumptuous character are still in part preserved in the Graeco-Roman theatres of Aspendos, Tauromenium, and various other cities. But it is uncertain how far they can be traced back into or beyond the Hellenistic period.[427] It is still keenly disputed whether the supporting walls for the stage-buildings, found in the ruins of different theatres, are really strong enough to bear the weight of two stories. As regards the doors which led from the back-wall on to the stage there is no positive evidence to be obtained from the existing ruins. But Pollux and Vitruvius state that they were three in number.[428]