A scale of six notes discovered in the Middle Ages and used to teach theory. The six notes correspond to the first six notes of the modern major scale. There were three hexachords:hexachordum durum: The six- note scale based upon G, containing B -natural, called B durum. ( G, A, B-natural, C, D, E)hexachordum naturale: The six- note scale based upon C, containing no B. ( C, D, E, F, G, A)hexachordum molle: The six note scale based upon F, containing B -flat, called B molle. ( F, G, A, B -flat, C, D) 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. a grouping of consecutive notes, by sixes rather than octaves (intervals of 8 notes). it was introduced in the 11th century and used up to the 17th