Abstract A harmonic pattern, termed the V–I schema, is shown to structure most of the second-reprise openings in Bach’s binary movements. This pattern combines cross-repertoire harmonic consistency with context-dependent prolongational variability, a juxtaposition that causes intractable contradictions to emerge within Schenkerian theory. The schematic aspect to early eighteenth-century tonality suggested by this feature is expanded to include two additional second-reprise openings, the V–vi schema and the in-III opening. A staple of most Baroque composers’ harmonic vocabulary, the V–I schema declined in the later eighteenth century, incompatible with the new prevalence of rounded binary form.