Custos
A small sign at the end of a staff line showing the pitch of the next line's first note.
What it means
The custos (Latin for "guardian") is an auxiliary sign used in the square notation of Gregorian chant and in white, and partly black, mensural notation. It appears only at the end of a staff system or immediately before a clef change, where it anticipates the pitch of the first note of the following line.
Unlike ordinary note signs, the custos is never sung or played: it serves purely as a reading aid so performers do not lose their place across a line break.
In the notation
Graphically the custos usually appears as a small, hook-shaped mark at the right edge of the staff, its position on the lines indicating the upcoming pitch. It was in common use until the seventeenth century and today survives mainly in modern editions of Gregorian chant such as the Editio Vaticana.