Tetrachord

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tetrachord

[′te·trə‚kȯrd]
(acoustics)
The basis of a variety of ancient musical scales, consisting of four notes, with an interval of a perfect fourth between the highest and lowest notes.

Tetrachord

 

in music, a succession of four pitches contained within the limits of a fourth. Tetrachords were the basis of musical modes and of the entire scale in ancient Greek music. The Greek names for diatonic tetrachords, just as the names for the corresponding modes, are still used in modern music theory, but they refer to modes with different interval structures.