The Collaborative International Dictionary
Tetrachord \Tet"ra*chord\, n. [L. tetrachordon, Gr. ?, from ? four-stringed; te`tra- (see Tetra-) + ? a chord: cf. F. t['e]trachorde.] (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.
Wiktionary
n. (label en music) Any set of four different pitch classes.
Wikipedia
In music theory, traditionally, a tetrachord (, ) is a series of four notes ("chords", from the Greek chordon, "string" or "note") separated by three smaller intervals that span the interval of a perfect fourth, a 4:3 frequency proportion. In modern usage a tetrachord is any four-note segment of a scale or tone row, not necessarily related to a particular system of tuning.
Usage examples of "tetrachord".
Then it touched itself on the mouth and the tip of its petaled tail and sounded the tetrachord again.
What class of creatures, he asked them, included both Tetrachord and Triad?
Then Tetrachord and Triad both turned to him and gave him an entirely different word.
He turned his back to Tetrachord and crouched, holding himself perfectly still.
Then Tetrachord made some adjustments at the keyboardlike row of fretted miniature guitar necks.
Then Tetrachord, still with a couple of arms around Triad, turned to his electronic zither and twisted some frets.
Triad and Tetrachord were off on one of their incomprehensible errands.
Lately Tetrachord and Triad had been moving equipment and cages elsewhere.
Jameson could move, the room was filled with hissing Cygnans, Triad and Tetrachord among them.
If it failed to make contact with Triad soon, then the union of Tetrachord and Triad would produce no young.
They became Klein murdering Tetrachord, and Triad streaking for the safety of the human compound.
With this as a measure he attempted to place the tones of the tetrachord, or Greek scale of four tones, which was the unit of their tonal system.
From them it appears that the basis of their scale was the tetrachord of four tones, placed at an interval of two steps and a half step.
The outside tones of the tetrachord remained fixed upon the lyre, but the two middle ones were varied for the purpose of modulation.
Didymus, and not Ptolemy, who proposed the tuning of the tetrachord which is now accepted as correct.