The Flat Seven (bVII)

A major chord a whole tone below the tonic. Whether it is a move at all depends on the key the line is read in: outside a major key it is a borrowing, inside a mixolydian one it is simply at home.

The rule, as the reader applies it

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

Where it fires: two times in two lines

Every line in this site’s corpus where the flat seven fires: the line, the key it is read in, the chords the move covers and their numerals.
Line Read in Chords Numerals Line from
Dm Bb7 C C major Bb7 bVII7 as printed in Borrowed chord, “Common borrowed chords” (Wikipedia)
C Em F G C Bb C C major Bb bVII constructed test case

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

Spotting it

The rule looks at one chord: a major chord whose root sits a whole tone below the tonic, in a key whose tonic chord is major, with at least one note outside that key. In C major it is Bb major, and Bb is the note outside. The numeral is bVII.

It depends on the key the line is read in

This is the move that most depends on the reading. C mixolydian is C major with its seventh degree lowered, so Bb major is one of its own chords. A line such as C Bb F C fits C mixolydian with no note outside at all, outscores C major, and is read in mixolydian; the Bb is then simply at home and nothing is flagged. The same happens in C Bb Ab Bb C.

A line that also holds chords the mixolydian scale lacks tips the other way. In C Em F G C Bb C the E minor and the G major both carry B natural, which C mixolydian does not hold, so C major wins outright and the Bb stands outside it. In Dm Bb7 C the Bb7 sits between ii and I; the line ties between C major, C mixolydian, D minor and D dorian, and the page reads it in C major by the tie order, where the Bb7 is outside and flagged. Its row reads bVII7.

Checking a reading

If the reader has not flagged a bVII you expected, look at the key named above the table. A mixolydian reading means the line holds nothing that rules the mode out, and adding the chord that does will usually change the answer.