The theatre was 111 m in diameter and was the largest and most important theatre in Ancient Rome;[2] it could originally hold between 11,000 and 20,000 spectators.[2][2] It was an impressive example of what was to become one of the most pervasive urban architectural forms of the Roman world. The theatre was built mainly of tuff, and concrete faced with stones in the pattern known as opus reticulatum, completely sheathed in white travertine. However, it is also the earliest dateable building in Rome to make use of fired Roman brick, then a new introduction from the Greek world.[3]