How did neumes help singers remember plainsong melodies?

In the context of musical notation, neumes functioned primarily as memory aids for singers who already knew the words and melody by heart.21

Early neumes, known as chironomic or staffless neumes, were graphic signs derived from ancient textual accents. They did not indicate precise pitches but instead provided a visual guide to the general rise and fall of the voice and vocal ornamentation. By acting as a continuous graph of pitch direction over the text, these signs helped performers recall the specific melodic contours of a piece during the semi-oral musical practices of the time.21

As notation evolved between the 10th and 12th centuries, neumes became "heighted" or "diastematic," meaning they were spaced vertically on the page to suggest more specific melodic lines. This development eventually led to the creation of the musical staff, which allowed singers to read unfamiliar melodies with greater precision.21