Research in ancient Roman architectural design has come increasingly to the view that geometry was often as important as metrication and proportion. The present paper examines the contribution of both geometry and arithmetic to the design of the four imperial fora in Rome, as well as the closely related Temple of Peace. An analysis of the Forum of Augustus-the best-known of the imperial fora-shows that it was designed according to a geometric model with a particular size utilizing a "base dimension" of 146 Roman feet. Analyses of the other fora show that the same geometric model-but with a base dimension of 150 feet-can be used to generate their basic spatial divisions and dimensions. The model accounts not only for straightforward and integral dimensions or proportions, but also for irrational and nonintegral proportions hitherto unexplained. The article argues that the mixture of integral, nonintegral, and irrational metrication was a deliberate aspect of the design process, in line with the Early Imperial propensity to combine rectilinear and curvilinear architectural forms. The article concludes with a suggestion that the model originated in the Etruscan ritual division of space, which was adopted by the Romans and later applied to an increasingly broad range of building types. The model may thus have served as a kind of template or cosmogram whereby each building designed on it could embody essential features of cosmic order. It gave to each Roman building its uniquely "Italic" flavor quite separate from Greek "cosmetics." If the model's application proves to be sufficiently broad, then the possibility is raised of developing a "unified design theory" for Etruscan and Roman architecture. These issues will be broached in future articles.

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