Why is a four-line staff still used for plainchant?

The four-line staff remains in use for plainchant primarily because it is the traditional notation system that has been used for this repertoire since the 13th century.1

The four-line staff was developed by Guido d’Arezzo around the year 1000 to provide a precise visual framework for pitch, allowing singers to move beyond learning melodies solely by rote. By placing neumes on lines and spaces, the system established clear pitch relationships, which were further defined by the use of clefs.23

While the five-line staff became the standard for polyphonic music by the 14th century to accommodate a greater range of voices and more complex textures, the four-line staff was sufficient for the monophonic nature of plainchant. Because plainchant notation is historically tied to this specific rectilinear style—featuring diamond-shaped notes and ligatures—the four-line staff continues to be the standard for this genre today.21