A group of 6 consecutive notes regarded as a unit for purposes of singing at sight—somewhat as the octave is in ‘movable‐doh’ systems. It was introduced (or perfected) by Guido d'Arezzo in the 11th cent. and was still widely current up to the 17th.
There were 3 different hexachords, the hard one beginning on G, the natural one beginning on C, and the soft one beginning on F. It will be realized that these overlapped in their range, and that a singer reading a piece of mus. might have to pass from one to another if its compass extended beyond one of those sets of 6 notes.
The names of the notes were taken from the opening syllables of 6 lines of a Lat. hymn, which syllables happened to ascend a degree with each succeeding line. These names were ut, re, mi, fa, sol, and la. Letter names were also then in use for the notes, but these were absolute names, as they are still, whereas the hexachordal names were relative to the group in use at the moment, as their successors the modern tonic sol‐fa names are relative to the key in use at the moment: the sol‐fa system (on its pitch side) may, indeed, be looked upon as a modernization of the hexachordal system, which served well in the period of simple modal mus. (see modes) but was incapable of application to the increasing complexities of a key system.
To the hexachords Guido added the device of The Guidonian Hand.
In the 20th cent. the term is applied to a coll. of 6 pitch classes considered either simultaneously or as a succession, esp. in reference to segments of 12‐note rows. Unconnected with medieval term.