The psychological ‘push’ of tritones within unstable-to-stable voice leading is an important source of energy in Shostakovich’s music. A sense of motion thus results from the re-stabilizing of harmonic instability. The ways in which Shostakovich resolves those tritones also accounts for something of the melodic-harmonic flexibility – yet also the steadfastly tonal nature – of his language. However, these observations do not realize the full potential of Yavorsky’s theory of modal rhythm, which, as a whole, concerns the unfolding of modes in time.2 For whilst voice leading and cadential progressions are by definition diachronic, the specifics of their temporal dimensions have not yet been described. Given an appropriate harmonic context, a tritone drives towards resolution at some point, but when? The temporal positions of changes within a pattern clearly define the duration of stabilities and instabilities, thereby having a significant impact upon the overall energy stream.