hexachords

Hexachord is a term from medieval music theory referring to a group of six consecutive pitches within a diatonic scale, spanning a fourth. It was the basic unit of the Guidonian hexachord system, used for teaching, sight-singing, and early notation. Each hexachord could be transposed to different pitch areas by using solmization syllables and by shifting the starting note.

There are three primary hexachords named for their starting pitch: the C-hexachord (C–D–E–F–G–A), the G-hexachord (G–A–B♭–C–D–E), and the D-hexachord (D–E–F–G–A–B). The C-hexachord is called the natural hexachord; the G-hexachord is the soft hexachord because it includes B♭; the D-hexachord is the hard hexachord because it contains B natural. The hexachords overlap to cover a wider range and to facilitate melodic and computational work within chant practice.

When singers moved from one hexachord to another, they used hexachordal mutation, adjusting the use of B-flat or B-natural to remain within the current hexachord. This system gradually declined with the adoption of modern staff notation and clearer tonal centers, though it remained influential in shaping medieval pedagogy and chant practice.

In modern theory, the term hexachord also appears in a broader sense to denote any six-note pitch-class set, and in some contexts to describe six-note combinations used in set theory and analysis.