A Dictionary of Music and Musicians/Seventh
SEVENTH. The intervals which contain seven notes comprise some of the most important chords in music, and such as have been peculiarly conspicuous in musical history. They are divided mainly into three classes—major sevenths, minor sevenths, and diminished sevenths; as
I. The major sevenths, as CB, FE, GF♯, are very harsh—in fact the harshest combination used in modern music except the minor second, such as BC. They are only endurable either when prepared and duly resolved, or when they result from the use of an appoggiatura or grace-note, or passing note. They occur most commonly as suspensions, resolving either up or down, while the rest of the chord is stationary, as at (a)
or with the condensed forms of resolution, when the rest of the chord moves simultaneously with the motion of the discordant note, as at (b)
Of these major sevenths there are several forms, but as they all have the same general principles of formation and treatment they do not require detailed consideration.
2. The minor sevenths are more individually characteristic. Of these the most important is the Dominant seventh, as at (c), for the key of C. The discordance of this combination is very slight.
By itself it is but little more harsh than some combinations which are universally accepted as concords, such as the minor sixth; but its harshness is increased by the addition of the other notes which fill up the harmony, as at (d), since the indispensable major third in the chord makes a diminished fifth with the seventh. Nevertheless its mildness has long been recognised, and it was used as early as the beginning of the 17th century with greater freedom than any other discord, by being relieved of the condition of being prepared. [See Harmony, vol. i. p. 674.] But the laws of its resolution continued, and still continue, more or less restricted. It naturally resolves into the Tonic chord; because its third is the leading note of the key and tends to the Tonic; its seventh naturally tends to the third of the Tonic chord, which is in the major divided from it only by the small interval of a semitone; and its root or bass note already supplies the fifth of that chord, which naturally acts as the connecting link between the two harmonies of dominant and tonic; so that all the vital notes of the Tonic chord are as it were predicted by its sounding, and consequently it is the most natural and forcible penultimate in cadences, in which it occurs with extreme frequency. [See Cadence; Harmony.] It is hardly necessary to point out that it can be resolved otherwise, since it so often plays a part in interrupted cadences; as for instance where the Tonic chord is supplanted by the chord of the Submediant (e); but it is in consequence of the very predisposition which it creates to expect the Tonic chord that interrupted cadences have such marked effect. [See Interrupted Cadence.] There is no other minor seventh in the key which can be accompanied by a diatonic major third; but there are two at least that can be obtained with one chromatic note in them, and these are so frequently used as if they belonged to the key that some theorists have agreed to affiliate them. These are the minor seventh on the supertonic with a chromatic major third, and the minor seventh on the tonic, in which the seventh itself is chromatic, as (f) and (g), in relation to the key of C. These are respectively the dominant sevenths of the Dominant and Subdominant keys, so that in any sense they lie very close to the principal key, and can resolve into it with the greatest ease; and they are often taken without preparation as distinct ingredients of its harmonic material without other reference to the keys to which they diatonically belong.
The minor seventh on the supertonic, with a diatonic minor third, is a chord which has much exercised theorists. It comprises the same notes as the chord which has been generally known formerly and even partially now as the Added sixth; and it is more often met with in the form from which that name was derived. But in whatever position, it has long been peculiar among discords for the variability of its resolution, since the note which would be the seventh if the supertonic were at the bottom of the chord, stands still in resolution almost as often as it moves downwards to the conveniently contiguous leading note of the key. For the various views entertained concerning this chord, see Harmony and Root.
3. The chord of the Diminished seventh ia a familiar combination both to theorists and musicians. It is in its complete form composed of a set of minor thirds, and this as much as anything gives it its notoriously ambiguous character, since any of its elements can be treated as the discordant note, with the result of leading to a different key in each several case. It is now commonly held to be the inversion of a minor ninth with the root note omitted. [See Diminished Intervals, vol. i. p. 448.]