A pentachord in music theory may be either of two things. In pitch-class set theory, a pentachord is defined as any five

pitch class In music, a pitch class (p.c. or pc) is a set of all pitches that are a whole number of octaves apart; for example, the pitch class C consists of the Cs in all octaves. "The pitch class C stands for all possible Cs, in whatever octave positio ...

es, regarded as an unordered collection . In other contexts, a pentachord may be any consecutive five-note section of a diatonic scale . A pentad is a five-note chord . Under the latter definition, a diatonic scale comprises five non-transpositionally equivalent pentachords rather than seven because the Ionian and

Mixolydian Mixolydian mode may refer to one of three things: the name applied to one of the ancient Greek ''harmoniai'' or ''tonoi'', based on a particular octave species or scale; one of the medieval church modes; or a modern musical mode or diatonic scal ...

pentachords and the Dorian and Aeolian pentachords are intervallically identical (CDEFG=GABCD; DEFGA=ABCDE). The name "pentachord" was also given to a musical instrument, now in disuse, built to the specifications of Sir Edward Walpole. It was demonstrated by Karl Friedrich Abel at his first public concert in London, on 5 April 1759, when it was described as "newly invented" . In the dedication to Walpole of his cello sonatas op. 3, the cellist/composer James Cervetto praised the pentachord, declaring: "I know not a more fit Instrument to Accompany the Voice" . Performances on the instrument are documented as late as 1783, after which it seems to have fallen out of use. It appears to have been similar to a five-string

violoncello The cello ( ; plural ''celli'' or ''cellos'') or violoncello ( ; ) is a bowed (sometimes plucked and occasionally hit) string instrument of the violin family. Its four strings are usually tuned in perfect fifths: from low to high, C2, G2, D ...

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