A series of six notes, with a semitone between the third and fourth, the other intervals being whole tones.

the six usable degrees of the modal scale, often used to organize Renaissance music.

a system of six notes separated by whole- or half-step. Guido of Arezzo (ca. 991-d. after 1033) assigned solmization syllables to each note of the hexachord: ut re mi fa sol la . (The only half-step comes between mi and fa.) Guido had three types of hexachords: the hard hexachord uses B natural and is built on G (G A B-natural C D E); the natural hexachord starts on C (C D E F G A); and the soft hexachord uses B flat and starts on F (F G A B-flat C D). To sight-read a melody, one picked the appropriate hexachord; if the melody extended beyond the boundaries of the first hexachord, one mutated to the next with a pivot-note. Because all of the half-steps are specified by the syllables mi-fa (or fa-mi), the system could be used to sight-read an unknown melody.