The Ancient Romans were able to construct their architecturally advanced arches and domes because of their development of pourable concrete and their skill in using special wooden forms, or molds, known as coffers. These construction advances enabled the Romans to efficiently build structures that previously would have been dependent upon a large number of pillars or support columns, and that would have required the transportation, hoisting and placement of large sections of cut stone.
A notable example of a Roman dome supported only by the structure's side walls is the Pantheon, which contains a support-free and uninterrupted open interior space under its rotunda. This open interior space measures more than 140 feet in diameter and height. The Pantheon's concrete dome was constructed by pouring wet concrete into the movable wooden molds, or coffers. When the wet concrete hardened, the wooden molds were removed, and the interior of the dome took on the appearance of having been constructed from recessed rectangular panels.