A
chord that consists of two superimposed minor thirds (forming a Diminished
triad) and
one major third (e.g. B–D–F–A). As a diatonic harmony its root may be the
supertonic of a minor key, or more rarely the seventh degree of a major key
(the above example could appear in A minor or C major); it is sometimes
interpreted as a rootless dominant major Ninth chord. Like the Diminished
seventh chord,
the half-diminished 7th may be reinterpreted through Enharmonic equivalence. Thus in E
minor the supertonic
half-diminished 7th is F–A–C–E, but when spelled F–G
–B–D it is known as the ‘Tristan’ chord and is resolved by Wagner to
the dominant of A minor. Even in diatonic form the chord is frequent in
Romantic harmony, the supertonic form being borrowed to enrich the major mode;
in Debussy it contributes to the atmosphere of Impressionism.
JULIAN RUSHTON