DICT.TW Dictionary Taiwan

 tet·ra·chord /ˈtɛtrəˌkɔrd/
 四度音階,四弦琴

 Tet·ra·chord n.  Anc. Mus. A scale series of four sounds, of which the extremes, or first and last, constituted a fourth. These extremes were immutable; the two middle sounds were changeable.
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