After a period in which he had concerned himself with ‘free’ dissonance, Weill began, in 1926–27, to return to a harmonic language based on tonality. Many other composers were doing something of the kind at that time, and, like him, were associating a return to tonality with references to popular dance rhythms, whether traditional or modern. But whereas such figures as Stravinsky, Hindemith, Prokofiev and Milhaud tended to reject traditional harmonic usages, Weill proceeded to develop a largely triadic harmony, paring down his vocabulary to the most basic of chords and progressions. Although the power and originality of that harmony have been widely acknowledged, the technical issues have received little or no attention, at least in any published form. What follows is a preliminary and necessarily selective survey.