The hexachord system was the major organizing system of tonality in the music of the Middle Ages. There were three hexachords, based on what is today the scales of C, F, and G. The interpretation of these hexachordal systems is not covered in most music classes, but an understanding of the hexachord system is necessary to be able to perform medieval music and early Renaissance music.

The C hexachord consists of the notes C, D, E, F, G and A. (See the exception rule below). The F hexachord consists of the notes F, G, A, B-flat, C and D; and the G hexachord consists of the notes G, A, B, C, D, and E. However, looking at medieval music, you will see that there are many more notes than can be covered in one hexachord; this is accomplished because the composer can move from hexachord to hexachord within the same piece of music, sometimes even in the middle of a phrase. Therefore, a piece of music can start out in the C hexachord, and once the notes go higher than A, the piece of music has now shifted to a new hexachord, either F or G. Where inside the phrase, and how extended the hexachord is, and how far-ranging the melody is, determine whether the note higher than A will be a B-flat or a B. Knowing where to shift hexachords is a matter of developing an ear for the transitions.

There is an exception to the rule that you must shift the hexachord outside of the six notes. If the hexachord is exceeded by only a half-step (in either direction), then the music is considered to stay within the same hexachord wherever possible. So you could have a piece that stays in the C hexachord and uses both the B-flat (on the top), and the B-natural (on the bottom). Many performing groups who specialize in medieval music are not aware of this rule, and therefore their performances are not as accurate as groups who observe this rule.