For centuries, chant melodies used in Christian liturgical worship existed only as sound, passed from singer to singer entirely through memory and oral transmission, with no reliable system for recording exactly which pitches a melody actually used. Musical notation, the written system that lets a piece of music be recorded and reproduced accurately by someone who's never heard it performed, wasn't handed down complete from some earlier source — it had to be invented, piece by piece, over a period of centuries, before Western music could be written down with any real precision at all.
Early notation recorded contour, not exact pitch
The earliest medieval attempts at musical notation, called neumes, indicated only the general rise and fall of a melody's contour above a line of chant text, showing a singer roughly where a melody moved up or down without specifying exactly which pitch, or how large an interval, was actually involved. This system worked reasonably well as a memory aid for singers who already knew a chant melody by ear and just needed a visual cue to help recall it, but it couldn't teach an unfamiliar melody to someone who'd never heard it before, since it simply didn't record enough precise information to reconstruct the actual pitches involved.
The staff turned rough contour into exact, reproducible pitch
The eventual solution, developed and refined particularly through the work traditionally credited to the medieval music theorist Guido of Arezzo in the eleventh century, was the musical staff: a set of horizontal lines and spaces, each representing a specific pitch, that let a neume's vertical position indicate an exact note rather than just a rough directional contour. This was a genuinely significant technical achievement — the staff, along with related developments like solmization syllables for sight-singing, meant a piece of music could, for the first time, be written down precisely enough that a trained singer who'd never heard the piece performed could still learn to sing it accurately directly from the page, a capability oral transmission and early neume notation alone had never reliably provided.
What we're still unsure about
The broad development from early contour-only neumes to the pitch-precise staff notation, and Guido of Arezzo's traditionally credited role in that development, are well documented through surviving medieval manuscripts and music theory writing. What's harder to establish with full certainty is exactly how accurately performances of the very earliest notated chant, before the staff's invention, actually preserved the original melodies across generations of purely oral transmission — since no audio record survives from that period, musicologists have to reconstruct probable early performance practice from later notated sources and comparative analysis, rather than being able to verify directly how faithfully melodies were actually preserved during the centuries they existed only by ear.
This sits inside Medieval & Renaissance Music, one of seven topics in Music History, one of four domains in Music, one of seventeen subjects the app can quiz you on.