Mode

Play the white keys of the piano from C up to the next C. Now play the same white keys from A up to the next A. Not one note is different, and yet the two scales do not sound alike: the first sounds bright and settled, the second darker.

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What changed is not the notes but where the semitones fall.

Where the semitones fall

A scale is a series of notes, one after the other, separated by tones and semitones, and its notes are numbered as degrees from the first one up (Lacerda 1966, ch. XX). On the white keys there are only two semitones, E–F and B–C. Everything else is a whole tone.

Starting on C, those two semitones sit between degrees III and IV (E–F) and between VII and VIII (B–C). Starting on A, the same two semitones sit between II and III (B–C) and between V and VI (E–F). Same semitones, different degrees.

That is what mode means: the way the tones and semitones are distributed among the degrees of a scale (Lacerda 1966, ch. XX). Med puts it in terms of character: the mode is the character of a scale, and it depends on where the tones and semitones fall and how they relate to the tonic (Med 1996, ch. XV).

Tonic and mode name a scale

A scale takes its name from two things: the note it starts on, the tonic, and its mode. C major is a scale that starts on C and is in the major mode; A minor starts on A and is in the minor mode (Lacerda 1966, ch. XX).

The two are independent. The major mode can start on any note, as long as the semitones stay between III–IV and VII–VIII. Starting on G with only white keys, the second semitone would land in the wrong place, between VI and VII (E–F). To put it back between VII and VIII, the F has to become F sharp.

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That is why the major scales on different notes need different accidentals: the accidentals are the price of keeping the mode.

And the other way round: the same tonic can carry different modes. C major and C minor share a tonic and differ in their third, sixth and seventh degrees. Med calls these the modal degrees, because they are where the mode shows. The third always differs; the sixth and seventh may or may not, depending on which form of the minor scale is used (Med 1996, ch. XV).

Seven ways, two in everyday use

If you build a scale with only white keys on each of the seven notes, the two semitones land in a different pair of places every time (Lacerda 1966, ch. XX):

Starting note	Semitones between
C	III–IV and VII–VIII
D	II–III and VI–VII
E	I–II and V–VI
F	IV–V and VII–VIII
G	III–IV and VI–VII
A	II–III and V–VI
B	I–II and IV–V

Seven different placements, so seven modes. Lacerda notes that two of them dominate music today: the major mode, the one starting on C, and the minor mode, the one starting on A. The other five he leaves for later in the book, with the modes of the medieval Church (Lacerda 1966, ch. XX).

The centre decides

Because several modes share the same notes, the notes alone do not tell you which mode you are hearing. What decides is which note the music treats as its centre: where the phrases come to rest, and where the melody ends.

Hear the passage

Hear the passage

Both melodies use only white keys. The first settles on C and sounds major; the second settles on A and sounds minor.

Where the books differ

The three books do not use the word in the same way.

Lacerda uses mode for every placement of tones and semitones, seven in all, of which two are in common use (Lacerda 1966, ch. XX, XXXV). Med, in the chapter on scales, defines mode as the character of a scale and says there are two, major and minor; the Dorian and its companions appear there as "ancient scales", and only later get a chapter of their own as liturgical modes (Med 1996, ch. XV).

Blatter goes further in another direction. For him, a mode is not just an order of tones and semitones: it is a collection of pitches in which some pitches have particular functions, such as the note melodies end on. He also warns that medieval musicians did not think of their modes as scales at all; laying them out as scales is a convenience for modern students, who already know scales (Blatter 2007, part 2, art. 7).

What comes next

The seven placements carry old names, taken from the modes of the medieval Church: Ionian and Aeolian, which became major and minor, and Dorian, Phrygian, Lydian, Mixolydian and Locrian. Several of those five survived in folk and popular music long after art music settled on major and minor.