In vindication of the use of the Inverted Seventh, it is necessary to explain, that, under certain conditions, the normal compass of all the Modes may be slightly extended. Every Authentic Mode may, by license, descend one degree below its Final: the Phrygian Mode may descend two. Every Plagal Mode may ascend to the sixth degree above the Final: the Hypolydian, and Hypomixolydian Modes, may, in addition, descend to the fifth below it.
Melodies confined strictly within the natural range of the Mode are called Perfect; those which fall short of it, Imperfect; those which exceed it, Superfluous. A melody which combines the entire compass of a Plagal with that of an Authentic scale, is said to be in a Mixed Mode.
Both in Plain Chaunt and Polyphonic Music, the Modes are used, sometimes, at their true pitch; sometimes, transposed a Fourth higher, (or Fifth lower), by means of a B flat at the Signature. No accidentals are permitted, in Plain Chaunt, except an occasional B flat, introduced for the purpose of correcting a Tritonus, or a False Fifth—the use of both these intervals being strictly forbidden, whether in disjunct or conjunct movement. [See Plain Chaunt [App. p.719 "Plain Song".] The Canto fermo, in Polyphonic Music, is as strictly subject to the laws of the Mode as a Plain Chaunt melody—which, in fact, it generally is: but, in the Counterpoint, the use of certain sharps, flats, and naturals, is sometimes directly enjoined, in conformity with precepts which will be found fully described elsewhere. [See Musica Ficta.]
In order to ascertain the Mode in which a Plain Chaunt Melody is written, observe the last note, which will, of course, shew the required Final. Should the compass of the Melody lie between that Final and its Octave, the Mode will be Authentic. Should it lie between the fifth above and the Fourth below, it will be Plagal. Should it extend throughout the entire range, from the Fourth below the Final to the Octave above it, it will be Mixed. Should there be a B flat at the Signature, it will indicate that the Mode has been transposed; and the true Final will then lie a Fourth below the written one. For example, the Plain Chaunt Melody, 'Angelus autem Domini' (for which see the article, Antiphon), has no B flat at the Signature. Its last note is G, the Mixolydian Final. Its compass lies between the Fifth above that note, and the Second below it. It is, therefore, in the Eighth, or Hypomixolydian Mode; and, as its range falls two degrees short of the full downward range of the scale, it belongs to the class of Imperfect Melodies.
To ascertain the Mode of a polyphonic composition, examine the last note in the Bass. This will be the Final. Then, should the range of the Canto fermo—which will almost always be found in the Tenor—lie between the Final and its Octave, the Mode will be Authentic. Should it lie between the Fifth above and the Fourth below, it will be Plagal. Should there be a B flat at the Signature, it will shew that the Mode has been transposed; and the true Final will then lie a Fourth below the last bass note. Thus, Palestrina's Motet, 'Dies sanctificatus,' has no B flat at the Signature. The last note in the Bass is G. The compass of the Canto fermo, as exhibited in the Tenor, lies, almost entirely, between that note and its Octave. The Motet, therefore, is in the Seventh, or Mixolydian Mode. The same composer's Missa Æterna Christi munera' has a B flat at the Signature, and is, therefore, transposed. The last note in the Bass is F, the Fourth below which is C—the Ionian Final. The compass of the Canto fermo, in the Tenor, lies between the transposed Final, and its Octave. Consequently, the Mass is in the Thirteenth, or Ionian Mode, transposed.
According to strict law, it is as necessary for the Canto fermo to end on the Final of the Mode as the Bass: but, when the last Cadence is a very elaborate one, it frequently contents itself with just touching that note, and then glancing off to others, after the manner of what we should now call a coda. The neophyte will always, therefore, find the last Bass note his safest guide, in this particular. [See Polyphonic Music [App. p.719 "Polyphonia".]
In order to accommodate the range of 'unequal' voices, it constantly happens, that the Treble and Tenor, are made to sing in an Authentic Mode, while the Alto and Bass sing in a Plagal one; and vice versa. In these cases, the true character of the Mode is always decided by the compass of the Canto fermo.
MODULATION is the process of passing out of one key into another.
In modern harmonic music, especially in its instrumental branches, it is essential that the harmonies should be grouped according to their keys; that is, that they should be connected together for periods of appreciable length by a common relation to a definite tonic or keynote. If harmonies belonging essentially to one key are irregularly mixed up with harmonies which are equally characteristic of another, an impression of obscurity arises; but when a chord which evidently belongs to a foreign key follows naturally upon a series which was consistently characteristic of another, and is itself followed consistently by harmonies belonging to a key to which it can be referred, modulation has taken place, and a new tonic has supplanted the former one as the centre of a new circle of harmonies.
The various forms of process by which a new key is gained are generally distributed into three classes Diatonic, Chromatic and Enharmonic. The first two are occasionally applied to the ends of modulation as well as to the means. That is to say, Diatonic would be defined as modulation, to relative keys, and Chromatic to others than, relative. This appears to strain unnecessarily the meaning of the terms, since Diatonic and Chromatic apply properly to the contents of established keys, and not to the relations of different shifting ones, except by implication.
Moreover, if a classification is to be consistent, the principles upon which it is founded must be uniformly applied. Hence if a class is distinguished as Enharmonic in relation to the means
