::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!). —