hexachord

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A sequence of six tones with a semitone in the middle, the others being whole tones, that was used in medieval music.

Century Dictionary and Cyclopedia

1. n. In Greek music: A diatonic series of six tones.
2. n. The interval of a major sixth.
3. n. An instrument with six strings.
4. n. In medieval music, a diatonic series of six tones, containing four whole steps and one half-step (between the third and fourth tones). The hexachord was an attempt to improve on the ancient tetrachord as a unit of musical analysis. The entire series of recognized tones, from the second G below middle C to the second E above it, was distributed among seven hexachords, beginning on G„, C‚, F‚, G‚, C, F, and G, respectively. Each hexachord was perfect in itself, and similar to every other; its tones were designated in order by the syllables ut, re, mi, fa, sol, and la. (See solmization.) Any given tone was designated both by its letter name and by its syllable name in full; middle C, for example, being known as C soi-fa-ut, etc. In actual singing the solmization and the singer's conception of the tones passed from one hexachord to another as far as necessary, the process of changing being called mutation. In contrapuntal writing the most perfect possible imitation was considered to be that which occurred between analogous tones of two hexachords. The hexachord system is doubtfully attributed to Guido d'Arezzo, in the eleventh century. It continued in use until, in the eighteenth century, the octave as a unit of analysis and the modern theory of key-relationship were recognized.

Wiktionary

1. n. music A series of six tones denoted with the syllables ut-re-mi-fa-sol-la separated by seconds, the only of which that is a minor second being mi-fa.

GNU Webster's 1913

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