Clef

Musical symbol used to indicate the written pitches of notes

A clef (from French: clef 'key') is a musical symbol used to indicate which notes are represented by the lines and spaces on a musical staff. Placing a clef on a staff assigns a particular pitch to one of the five lines or four spaces, which defines the pitches on the remaining lines and spaces. The three clef symbols used in modern music notation are the G-clef, F-clef, and C-clef.

From Wikipedia, under CC BY-SA. More on occurri.