Substitute chord
A chord that stands in for a function.
What it means
A substitute chord (German Ersatzklang) — more often called Vertreter or Vertretungsklang — is a chord that takes the place of a principal function and assumes its role. Functional theory describes every chord of a key as a principal function or as its substitute; the substitutes are sonorities that share two notes with the principal function and therefore act similarly.
Musical use
Two kinds of substitution are especially important. The relative lies a minor third below a major sonority or a minor third above a minor sonority: in C major, A minor substitutes for the tonic as the tonic relative (vi), D minor for the subdominant as the subdominant relative (ii). The mediant lies a major third above a major sonority: E minor appears in C major as the tonic mediant (iii). Both share two notes with their principal function and therefore remain related.
The best-known use of a substitute chord is the deceptive cadence. The dominant does not lead to the expected tonic but to the tonic relative — the sonority is related enough to act as a continuation but different enough to refuse the ending. Equally familiar is the subdominant relative before the dominant, which makes the cadence softer and more colourful than the subdominant itself.
In jazz the idea of substitution has been developed in another way, for instance in the tritone substitution, in which a dominant is replaced by another at a tritone's distance, because both contain the same tritone.
The substitute chord is to be distinguished from the altered chord: the latter alters a sonority chromatically, the former takes its place as a sonority belonging to the scale.
In functional theory it is above all the relatives and mediants that count as substitutes for the principal functions: the tonic relative can replace the tonic, the subdominant relative the subdominant. These sonorities share two notes with the chord they stand in for, which is why they act similarly and yet bring new colour. In jazz one speaks of substitution.
Performance practice
Listen for which function a secondary chord stands in for — this determines its shaping. With the deceptive cadence, give the substitute chord the weight of a surprise, not that of an ending. And use substitutions in your own harmonizing to accompany a melody more colourfully.