Semiology (Gregorian chant)
Semiology (from Greek σημεῖον sēmeion, "a sign, a mark") is a branch of Gregorian Chant research. Semiology refers specifically to the study of the neumes as found in the earliest fully notated manuscripts of Gregorian Chant, the oldest of which have been dated to the 9th century.
Source: Wikipedia — Semiology (Gregorian chant) (CC BY-SA 4.0)