Modal interchange, and the colour that comes from the parallel
Chords taken from the parallel key, which change a degree's colour without changing centre; the minor fourth degree in a major piece is the most frequent one in Brazilian song and the easiest to hear.
A song in C major, and in the middle of it an F minor chord. F minor is not in the field of C major; it is in the field of C minor. The song has not changed key: it borrowed a chord from the parallel for one bar and gave it back. That is modal interchange, and it is one of the most frequent and most recognisable devices in Brazilian song.
Where the borrowed chords come from
The parallel minor of C major is C minor, and its field has seven chords, nearly all of them different from those of C major:
Any of them can be borrowed. In practice four are used constantly, and each has a name that says what happened to the degree:
- iv — F minor: the fourth degree, which was major, turns minor.
- ♭VI — A flat major: the sixth degree, with its root lowered.
- ♭VII — B flat major: the seventh degree, lowered and major.
- ♭III — E flat major: the third degree lowered.
The flat before the numeral is not decoration: it says the root fell a semitone against the diatonic degree. The sixth degree of C major is A; the borrowed one is A flat, and the spelling carries that.
The minor fourth degree
The commonest borrowing is the minor fourth degree, and it is worth knowing by sound before by name. Play F major followed by C major, and then F minor followed by C major:
The difference is one note: the A of F major becomes A flat in F minor, and that A flat falls a semitone to the G of the C chord. It is a plagal cadence with one darkened note, and the effect is of farewell, of a sigh. It is in countless songs, almost always just before the end of a phrase.
The lowered sixth and seventh
A flat major and B flat major, in C major, usually turn up together, in a descent by whole tones to the tonic:
It is a rock and film-score sonority, open and with no leading tone: B flat major reaches the tonic from below by a whole tone, not by a semitone. The ♭VI also serves as a surprise chord after the dominant, in place of the sixth degree in a deceptive cadence, with the same effect of a phrase gaining another piece and a darker colour.
The function stays the same
A borrowed chord inherits the function of the degree it replaces. F minor is subdominant, like F major; A flat major plays the sixth degree's role; B flat major plays a role close to the dominant's, without the leading tone. That is why borrowing does not disorganise the phrase: the sequence of functions runs on as before, and only one chord's colour changed.
It is also why borrowing is not modulation. The tonic is still C, and the chord after the borrowed one is usually diatonic again. The ear registers a shadow passing, not a change of place.
What to take from here
Modal interchange is taking a chord from the parallel minor, for a bar, without changing tonic. The most used are the minor fourth degree, the lowered sixth and seventh and the lowered third, and each inherits the function of the degree it replaces, so the phrase stays the same in a different colour. The flat in the numeral says the root fell, and the flat spelling says where the chord came from.