The Dominant a Tritone Away (bII7)

A dominant seventh whose root sits a semitone above the chord it moves to — Db7 into C. Its root is a tritone from the key’s usual V, and the reader finds it by that semitone slide.

The rule, as the reader applies it

  1. In any key: a major chord with a minor seventh, with at least one note outside the key, followed by a chord whose root is a semitone below it.

Where it fires: one time in one line

Every line in this site’s corpus where the dominant a tritone away 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 Dm G7 Db7 C C major Db7 C bII7–I constructed test case

The move as declared here was read from Tritone substitution, “Jazz / Pop” (Wikipedia), which prints it as Db7–C in C major, a tritone away from G7–C.

Spotting it

The reader needs a dominant seventh chord, a major triad with a minor seventh on top, with at least one note outside the key, followed by a chord whose root sits a semitone below it. In C major the usual case is Db7 moving to C. Db7's root is a tritone away from G, the root of the key's own V7, which gives the move its name here; the numeral is bII7.

In the table

In C Dm G7 Db7 C the ordinary V7 is heard first, and the Db7 then slides down a semitone to the tonic. Its row names the notes outside C major, and the span covers the Db7 and the C it moves to. C Db7 C is the same move with nothing before it.

What it needs

The rule asks for the seventh. A plain Db major triad in the same place, as in C Db C, carries notes outside the key but is not a dominant seventh, so it gets its numeral, bII, and no label. The slide matters too: the chord after the dominant has to sit a semitone below it. The rule does not ask what key the line is in, only that the dominant carries a note from outside it, so the move can turn up in front of any chord a line reaches, not just the tonic.

Where the name comes from

The source article describes the dominant a tritone away from V7 in the context of jazz and popular music, with Db7 to C in C major as its example. This site takes only the pattern that can be read in a line of symbols: a dominant seventh sliding down a semitone into the next chord.