Chord Symbols and Terminology
Half-diminished seventh chords are often symbolized as a circle with a diagonal line through it, as in Cø. It also can be represented as m7♭5, -7♭5, m7(♭5) etc.
The terms and symbols for this chord break expectations that derive from the usual system of chord nomenclature. Normally a symbol like "Bdim" indicates a diminished triad and "B7" indicates a major triad plus a minor seventh. Thus one would expect the term "Bdim7" to indicate a diminished triad plus a minor seventh. Instead, it means a diminished triad plus a diminished seventh. To make this distinction clear, the term "half-diminished" and the ø symbol (ø) were invented. Since the term dim7 (as in Bdim7) meant something else, the accurate but unwieldy term "minor seventh flat five" (as in Bø7))( Play) came to be used.
Despite the appearance of the word "diminished" in the name of this type of seventh chord, its sound differs considerably from that of a diminished seventh chord. In fact, the only sonic connection between the two chords is the single diminished triad found in the half-diminished seventh chord. As composer-theorist Milton Babbitt has astutely pointed out, the "half-diminished" seventh chord should be called the "one-third" diminished seventh chord. ... Whatever its deficiencies might be in the label department, however, the half-diminished seventh chord is in many respects the star of the seventh chord harmonic cast. Many songs in the classic American popular song repertoire reserve it for their most intensely expressive moments.Jazz musicians typically consider the half-diminished chord as built from one of three scales: the seventh (Locrian mode) of the major scale, the sixth mode of the melodic minor scale (the latter scale is nearly identical to the Locrian mode, except that it has a natural 9 rather than a ♭9, giving it a somewhat more consonant quality), or the "half-whole" diminished scale (see octatonic scales.) See: chord-scale system.
The "Tristan chord" is sometimes described as a half-diminished seventh chord; however, the term "Tristan chord" is typically reserved for a very specific harmonic function, especially determined by the order of the notes from bottom to top, and sometimes even the way the chord is spelled (e.g. is it G♭ or F♯?).
Read more about this topic: Half-diminished Seventh Chord
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