The concept, proposed by Rameau (Traité de l’harmonie, 1722), that chords of the 9th and 11th, among others, arise from a 7th chord by placing a ‘supposed’ bass one or two 3rds below the Fundamental bass. For instance, in the chord f–a–c'–e'–g'–b' the fundamental bass is c', while the ‘supposed’ bass is f. The doctrine of chords by supposition was adopted and modified by Roussier, Marpurg and others; A.F.C. Kollmann claimed to confute it by averring that it was theoretically simpler to treat Rameau’s ‘supposed’ bass as the fundamental and to regard the 9th and 11th, following Kirnberger, as structurally inessential transient notes.
Rameau, in calling his concept ‘supposition’, extended a sense in which the word had been used to describe notes of a melody that do not belong to the concurrent harmony; see, for instance, the definition of ‘supposition’ in J.G. Walther’s Musicalisches Lexicon (1732).
See also Ornaments, §7.
MICHAEL KASSLER