SUPPOSITION, in music, according to Rousseau, Supposition has two senses.
1. When several notes rise or descend diatonically in one part upon the same found in another, then these diatonic notes cannot all form a harmony, nor enter at once into the same chord: there are some of them, therefore, which must be reckoned as nothing; and it is these notes foreign to the harmony which are called notes by supposition.
The general rule is, when the notes are equal, that all those which are struck in the perfect time should carry harmony; those which pass during the imperfect time are notes of supposition, which are only placed there on account of the melody, and to form gradations conjoined. The reader will see what is meant by conjoined or disjoined gradations, by consulting MUSIC, art. 50. But, to render the matter perspicuous, let it be understood, that, in gradations conjoined, the highest found of the inferior degree forms likewise the lowest of the superior; whereas, in such as are disjoined, the series from beginning to end continually rises or descends by some one interval or another; and in this case the highest note of the lowest degree will not begin the succeeding series, which must commence at least a tone or semitone above it. Let it be observed, that by perfect and imperfect times we would be less understood to mean the principal divisions of the bar, than even the aliquot parts of each division. Thus, if there were two equal notes in the same time, or, in other words, two aliquot parts in the same principal division of the bar, it is the first of these notes or parts which carries the harmony; the second is a note of supposition. If the time consists of four equal notes, the first and third carry harmony, the second and fourth are notes of supposition, &c.
Sometimes this order is reversed; the first note is passed over by supposition, and the second carries harmony: but in that case the duration of the second note is generally increased by a point, at the expence of the first.
All this constantly supposes a diatonic procedure by conjoined gradations; for when the degrees are disjoined, there is no such thing as supposition, and all the notes ought to enter into the chord.
2. Those are also called chords by supposition in which the continued bass adds or supposes some found beneath the fundamental bass; which occasions such chords always to occupy a larger compass than the extent of the octave.
The dissonances of chords by supposition should always be prepared by syncopations, and resolved in descending diatonically upon some found of a chord under which the supposed bass may be recognized as a fundamental, or at least as a continued bass. For this reason, chords by supposition, if thoroughly examined, may all of them pass for mere suspenses.
There are three kinds of chords by supposition; all of them are chords of the seventh. The first, when the superadded found is a third below the fundamental; such is the chord of the ninth: if the chord of the ninth is formed by the mediant superadded beneath the chord containing the sensible note in the minor mode, in that case the chord takes the name of the fifth redundant. The second kind is, when the found supposed is a fifth beneath the fundamental,
Suppository as in the chord of the fourth, otherwise called the eleventh; if the chord contains the sensible note, and the tonic be supposed, the chord takes the name of seventh redundant. The third kind is that where the supposed sound is beneath a chord of the seventh diminished: if it is a third below, that is to say, if the sound supposed be the dominant, the chord is called a chord of the second minor and third major; it is very little in practice: if the sound superadded be a fifth below, or if that sound be the mediant, the chord is called a chord of the fourth and fifth redundant; and if it is a seventh below, that is to say the tonic itself, the chord takes the name of the sixth minor, and seventh redundant. With respect to the inversion of these different chords, where the supposed sound is transferred to the higher parts, as it is only admitted by a licence, it should not be practised without choice and circumspection. At the word ACCORD, in Rousseau's Musical Dictionary, will be found all those which are tolerable.