A 1999 survey of some 3,000 travertine building elements and other objects in Ostia demonstrated that this material was used for a variety of purposes, including as cornerstones. Two functions for cornerstones are proposed here. While the cornerstones in Ostian buildings have often been regarded as buffer stones, there are strong indications that they were boundary stones used for the layout of insulae. Cornerstones in small streets, constricted parts of streets, projecting buildings, and entrances of buildings were used as warning signals for passing traffic.