↑ The first explicit prohibition of that interval seems to occur with the "development of Guido of Arezzo's hexachordal system which made B flat a diatonic note, namely as the 4th degree of the hexachordal on F. From then until the end of Renaissance the tritone, nicknamed the 'diabolus in musica', was regarded as an unstable interval and rejected as a consonance" (Sadie, Stanley [1980]. "Tritone", in The New Grove Dictionary of Music and Musicians, 1st ed. MacMillan, pp. 154–5. ISBN 0-333-23111-2. See also Arnold, Denis [1983]. "Tritone", in The New Oxford Companion to Music, Volume 1: A-J. Oxford University Press. ISBN 0-19-311316-3). During the Baroque và Classical eras, the interval came to be accepted, though in a specific, controlled way. It is only during the Romantic era and in modern classical music that composers have used it freely, exploiting the evil connotations with which it is culturally associated.
