Modal transposition

The church modes are usually presented on the white keys: Dorian from D to D, Phrygian from E to E, Lydian from F to F. That is the simplest way to show them, because none needs an accidental. But it is only the model. Just as the major scale can start on any note, natural or altered, so can the other modes, as long as the placing of tones and semitones proper to each is kept (Lacerda 1966, ch. XXXV).

Transposing a mode means writing the same mode at another pitch (Med 1996, ch. XXIX). The pattern of tones and semitones stays; the starting note changes. D Dorian and C Dorian are the same mode, a major second apart:

Hear the passage

Hear the passage

The hard part is knowing which accidentals to use. There are several procedures, and Med recommends one as the simplest and quickest (Med 1996, ch. XXIX).

Every mode lives on a degree of the major scale

In the C major scale, every note is the final of one of the modes: Dorian starts on the second degree, Phrygian on the third, Lydian on the fourth, Mixolydian on the fifth (Med 1996, ch. XXIX). Ionian is the major scale itself, and Aeolian, starting on the sixth degree, is the natural minor scale.

Mode	Starts on degree of the major scale
Ionian	I
Dorian	II
Phrygian	III
Lydian	IV
Mixolydian	V
Aeolian	VI
Locrian	VII

This does not hold only for C major. Every major scale has the same relationships, so Dorian can be found in any major scale by taking its second degree as the final (Med 1996, ch. XXIX). Transposing a mode therefore becomes a question about major scales, which you already know how to answer: of which major scale is this note the right degree?

Given the first note

The commonest case. Build the Dorian mode starting on A.

Dorian starts on the second degree. A is the second degree of G major. So A Dorian uses the notes of G major, from A to A, with the key signature of G major (Med 1996, ch. XXIX):

Hear the passage

To check, look at what defines Dorian: a minor third between the final and the third degree, and the characteristic interval, the major sixth, between the final and the sixth degree. A–C is a minor third; A–F sharp is a major sixth. The mode has arrived intact.

Given the interval of transposition

Transpose the Phrygian mode up a major second.

The model Phrygian starts on E, and a major second above E is F sharp. Phrygian starts on the third degree, and F sharp is the third degree of D major. So F sharp Phrygian uses the notes of D major (Med 1996, ch. XXIX):

Hear the passage

The check: a minor third from F sharp to A, and Phrygian's characteristic minor second, from F sharp to G.

Given the key signature

Build the Lydian mode with five sharps.

Five sharps is the key signature of B major. Lydian starts on the fourth degree, and the fourth degree of B major is E. So Lydian with five sharps is E Lydian (Med 1996, ch. XXIX):

Hear the passage

A major third from E to G sharp, and Lydian's characteristic augmented fourth, from E to A sharp.

The three questions are the same sum worked from different ends: starting note, degree of the mode, major scale. Given two of these, the third follows.

The other route: altering the scale with the same tonic

There is a second procedure, starting from a different idea. Each mode, except Ionian and Aeolian, differs from the major or minor scale on the same tonic in one note only, the note of its characteristic interval. To build the mode, write the scale of the same kind, minor for Dorian and Phrygian, major for Lydian and Mixolydian, and bring the characteristic interval into it (Med 1996, ch. XXIX).

Build the Dorian mode on F. Dorian is a minor-type mode, so start from F natural minor, with four flats, and raise the sixth degree, from D flat to D, to get the major sixth:

Hear the passage

By the first route, F is the second degree of E flat major, and F Dorian is written with three flats, with no accidental on the staff:

Hear the passage

The notes are the same, and so is the sound. Only the writing changes (Med 1996, ch. XXIX). Med also notes that the accidental used to alter the scale only touches the end of the key signature: it either adds one more accidental or cancels the last one (Med 1996, ch. XXIX). Here the natural on D cancels the fourth flat of F minor, and what is left is the three flats of E flat major.

And there is a limit: you cannot reach a major-type mode by altering a minor scale, nor the reverse (Med 1996, ch. XXIX). The third does not change in this process, and the third is what decides the type.

Which key signature to use

Both routes are correct, and each way of writing shows something.

• The key signature of the major scale the mode comes from keeps the staff clean. It is the easiest to read and play, because the signature already carries every note.
• The key signature of the key with the same tonic, with an accidental on the characteristic note, puts on show what makes the mode what it is. Someone reading F Dorian with four flats and a D natural sees at once that it is "F minor with a major sixth".

Both turn up in practice. A score in F Dorian with three flats is not in E flat major, and one with four flats and a D natural does not have a wrong accidental. In both cases what tells you the mode is the final, the note the music treats as its centre.

What comes next

Transposing a mode is transposing a scale with one extra step: remembering which degree it lives on. The same reasoning helps with transposition by clef substitution, where the notes keep their place on the staff and change their names.