The half-diminished seventh is less common than the diminished seventh as a chromatic chord, although it has similar origins and function. The half-diminished seventh as a diatonic chord is discussed in embellished diatonic chords
It is frequently used in salon-type pieces to add a bit of colour - in this extract from Elgar's 'Salut d'Amour' it functions as a secondary dominant. The falling chromatic line from C# to C to B is an important part of the schmaltzy effect.
A diminished seventh at the beginning of Richard Wagner's overture to the opera Tristan and Isolde has become perhaps the most discussed chromatic chord of all time. The tonally ambiguous opening half-diminished seventh (or Tristan chord) initially resolves onto an E7 chord , but the same sequence is then repeated three times at different transpositions.
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