noun. A series of four diatonic tones encompassing the interval of a perfect fourth.

noun. In music: An instrument with four strings.

noun. The interval of a perfect fourth.

noun. A diatonic series of four tones, the first and last of which are separated by a perfect fourth.

noun. 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.

noun. In music a tetrachord is any set of four different pitch classes.

"Now then, these intervals of tones and semitones of the tetrachord are a division introduced by nature in the case of the voice, and she has defined their limits by measures according to the magnitude of the intervals, and determined their characteristics in certain different ways."

dyad  hexachord  interval  trichord