 The first explicit prohibition of it seems to occur with the development of Guido of Arezzo's hexachordal system, who suggested that rather than make B a diatonic note, the hexachord be moved and based on C to avoid the F–B tritone altogether.Later theorists such as Ugolino d'Orvieto and Tinctoris advocated for the inclusion of B.From then until the end of the Renaissance the tritone was regarded as an unstable interval and rejected as a consonance by most theorists.