A clef is the sign at the start of a stave — the five lines of written music. It tells you which notes live on which lines, so you know how high or low the music is. The two you will meet first are the treble clef (the swirly one, often for higher notes, or the right hand on a piano) and the bass clef (the one that looks a bit like a comma with two dots, for lower notes). There are others, such as the alto clef, but learners meet those later. The word came into English from French, and before that from Latin clavis, a key — it unlocks the pitch of the notes. Though actually, when English speakers talk about the key of a piece of music, that key is a different word with a different history. The two ideas sit near each other, but they are not the same family.