One of the hidden but nevertheless crucial elements in the development of the architectural vocabulary of imperial Rome was the use of metal fittings, such as clamps, dowels, and tie bars. These elements became particularly important when marble was introduced into Rome as a major building material. The growth of the marble trade during the imperial period created an environment where, on the one hand, Greek classicism provided models of trabeated structures that had developed in the marble-rich areas of the Aegean and, on the other, concrete provided the potential of creating new types of interior spaces. The marriage of these two was ultimately made possible by the hidden metal fittings that allowed the concrete vaults to be securely attached to the marble support structure.

Metal clamps and other experimental uses of metal in architectural contexts had appeared in Greece by the fifth century b.c., but they only appeared in Rome at the end of the second century b.c. when travertine and imported Greek marbles began to be used there. The Romans had access to the Etruscan iron resources from Elba and Populonia from the mid-third century b.c., so the late development of the use of iron for clamps seems to have more to do with the supply of stone than with the supply of metal. The earliest datable use of clamps in Rome are the iron pi clamps used to attach the travertine facing of the Metellan rebuilding of the Temple of Castor in the Forum just after 117 b.c. Other examples occur sporadically during the late Republic such as in the round temple in the Forum Boarium (c. 100 b.c.), where iron pi clamps were used to attach the Pentelic marble facing of the cella to the travertine backer blocks.