New fieldwork at the Sanctuary of the Great Gods on the Greek island of Samothrace has uncovered the remains of flat arches in the Doric frieze of the Stoa, a long portico built in the second quarter of the third century BCE. The keystone frieze was used prominently in large-scale building in Rome and exemplifies how Roman architecture creatively combined Greek trabeated aesthetics with the structural potential of the arch. The keystone frieze discovered on Samothrace, however, predates by one and a half centuries examples known in Italy. This article queries whether flat relieving arches were more widely deployed in Greek architecture but have gone overlooked. Other early Hellenistic buildings on Samothrace tested the limits of stone spans and set the stage for structural innovation. The Stoa reveals a decisive transition between the relieving devices based on cantilevers used in fifth- and fourth-century BCE Athens and the wider adoption of plate-bande construction in late Republican Rome.