You are right Jerome and I apologize for not having been entirely clear or I must avow not entirely kowledgeable On the one hand however I am not so sure about the statistical importance of three vs four part writing in the 14th century unless I am mistaken Vitry often wrote in four parts and Machaut did the same for his Mass and many other works On the other hand what seems to become a norm is that even five parts or more appears to involve two or more parts of the same tessitura let s say that four tessituras writing may have become one of the norms Machaut s mass on reflection seems to be in four parts but three tessituras You are right and the situation is complex instrumental quartets of the early 16th century e g in Virdung involved only three tessituras etc Let s say that all this deserves comments in the article that we may have in mind rather than Barber shop quartets singing in unison