The Flat Three (bIII)

A major chord a minor third above the tonic of a key whose home chord is major — Eb in C. Two of its three notes are outside the key, and the parallel minor key holds all three.

The rule, as the reader applies it

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

Where it fires: one time in one line

Every line in this site’s corpus where the flat three 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 Eb F C C major Eb bIII constructed test case

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

Spotting it

The reader flags bIII on one chord: a major chord whose root sits a minor third above the tonic, in a key whose own tonic chord is major, with at least one note outside that key. In C major the chord is Eb major, spelled Eb, G and Bb. Only G belongs to C major; Eb and Bb are the two notes the fit column names.

In the table

The numeral is bIII: upper case for the major triad, and a flat because the root is a semitone below the key's own third degree. In C Eb F C the chord sits between the tonic and IV, and it is the one row with anything outside the key. The line ties between C major and C mixolydian, and neither scale holds Eb, so the move is flagged whichever of the two you read it in.

Borrowed before anything else

Eb major is also a chord a third away from C with the tonic's own quality, which is the shape of a chromatic mediant. The reader checks borrowing first: every note of Eb major belongs to C minor, the parallel key on the same tonic, so in C Eb C the chord is read as bIII and the chromatic mediant rule stays silent. That is the order the source for the chromatic mediant gives: rule out borrowing, then a chord leaning on the next one, and only then call it a mediant.

What is not flagged

In a minor key the same chord is part of the key. Cm Eb Cm reads in C minor, the numeral is still written bIII because every numeral here is counted from the major scale, but nothing is outside the key and the move does not fire.