Everyday economic practice in the Roman Empire tended to operate on the interface between the known and the unknown (and the knowable and the not-knowable). While the unprecedented combination of concentrated wealth and sharply increased interaction over longer distances in many places created all kinds of economic opportunities at a variety of economic scales, they also presented economic actors with a paradigm of communicative and coordinative challenges, as many opportunities depended on interactions outside the everyday living environment or involving (relative) strangers. This paper starts from the idea that the very shape of the urban environment, and thus architecture, presented a range of opportunities to mitigate these challenges, and to facilitate encounter, and it will argue that several developments in urban architecture essentially had the effect of opening up and standardizing urban environments to such an extent that encounters between relative strangers became increasingly straightforward, thus lowering transaction costs in many spheres of economic practice.