The Flat Six (bVI)

A major chord built on the note a minor sixth above the tonic, in a key whose home chord is major. Two of its three notes are outside the key, which makes it one of the easiest moves to catch.

The rule, as the reader applies it

  1. In a key whose tonic chord is major: a major chord a minor sixth above the tonic, with at least one note outside the key.

Where it fires: three times in three lines

Every line in this site’s corpus where the flat six fires: the line, the key it is read in, the chords the move covers and their numerals.
Line Read in Chords Numerals Line from
C Bb Ab Bb C C mixolydian Ab bVI as printed in Borrowed chord, “Common borrowed chords” (Wikipedia)
Ab Bb C C mixolydian Ab bVI as printed in The Flat VI-Flat VII-I Cadence (StudyBass)
C Ab C C major Ab bVI as printed in Chromatic mediant (Wikipedia)

The move as declared here was read from Borrowed chord, “Common borrowed chords” (Wikipedia), which prints it as bVI in C major: Ab–C–Eb.

Spotting it

The rule looks at one chord. Its root sits a minor sixth above the tonic, its triad is major, at least one of its notes lies outside the key, and the key's own tonic chord is major. In C major that chord is Ab major, spelled Ab, C and Eb, and two of its three notes are outside the key, so the fit column names both Ab and Eb.

In the table

The numeral on its row is bVI. In C Ab C the chord sits alone between two tonic chords. In C Bb Ab Bb C it sits between two Bb chords, and that line reads in C mixolydian rather than C major, because the Bb chords fit the mixolydian scale; the Ab still has two notes outside it, and the move is flagged there too. The same line also fires the three-chord move bVI–bVII–I on its last three chords.

Not a chromatic mediant here

Ab major a third from C major has the tonic's own quality, and the source for the chromatic mediant lists it among the four chords of that kind in C. It also notes that the chord is a borrowed one, and the reader settles the overlap by checking borrowing first: every note of Ab major belongs to C minor, the parallel key, so the chord is read as the flat six and the chromatic mediant rule does not fire.

What is not flagged

In a minor key the chord on the lowered sixth is part of the key. Am F G Am reads in A minor with F as bVI and nothing outside, and the move stays silent, because its rule only applies where the tonic chord is major.