Why were hexachords important for singers in the Middle Ages and Renaissance?

In the Middle Ages and Renaissance, the hexachord—a six-note pattern—was essential for singers because it provided a consistent framework for pitch relationships. Devised by Guido of Arezzo, this system used solmization syllables (ut, re, mi, fa, sol, la) to help singers orient themselves while sight-reading or learning melodies.1

The system's primary value lay in its fixed internal structure: every hexachord contained exactly one semitone, located between mi and fa. By using three varieties of hexachords (natural, hard, and soft), singers could navigate different musical contexts—such as the inclusion of $B♭$ or $B♮$—while maintaining the same relative pitch intervals.1

When a melody exceeded the range of a single hexachord, singers employed a technique called mutation to shift between overlapping hexachords at pivotal notes. This practical device allowed them to apply the same syllable patterns to any series of notes, ensuring they always knew exactly where to place the semitone interval.1