The 'rhythmicisation' of plainsong is a subject which experts approach with great temerity. We think we know, however, that in the Renaissance the performance of plainsong was far removed from our modern conception, which tends to refer to the neume notation of the earliest manuscripts. The traditions of chant performance had clearly become very stylised by the 15th century - quite apart from the development of whole families of melodic variants, derived originally from variable oral transmission of models nearer the sources. The German chant 'dialect', with its consistent and characteristic adaptation of standard melodies is well preserved in the Passau sources, even though these are no longer regarded as authoritative for the repertoire of the Imperial Court Chapel and the works of Isaac. To draw an analogy with the 'reforms' of Palestrina and his colleagues 100 years later in Rome is to go too far, but authorities seem to agree on a slow and rhythmical performance of the chant, some insisting on a pulse much slower than ours. Our interpretation seeks to recreate a pulse to which that of the polyphony can be related.