A custos is a staff symbol that appears at the end of a staff line
with monophonic musical contents (i.e., with a single voice). It
anticipates the pitch of the first note of the following line and thus
helps the player or singer to manage line breaks during performance,
thus enhancing readability of a score.
Custodes were frequently used in music notation until the 16th
century. There were different appearences for different notation
styles. Nowadays, they have survived only in special forms of musical
notation such as via the editio vaticana dating back to the beginning
of the 20th century
Music Dictionary