A clef is a symbol placed at the beginning of a musical staff (those five horizontal lines where musical notes are written) that tells musicians which notes the lines and spaces represent. Without a clef, you'd be looking at notes but wouldn't know if they were high or low.

The most common clef is the treble clef, which looks like a fancy, curled ampersand. It tells you that the notes are in a higher range, like those you'd play with your right hand on a piano or sing in a high voice. The bass clef looks like a backward C with two dots, and it indicates lower notes, like those a cello or tuba would play.

Think of a clef like the key on a map: the map is useless without it. Similarly, written music needs a clef so musicians know exactly which pitches to play. When a flute player and a tuba player both read music marked with their respective clefs, they're actually playing very different notes, even though the symbols might look similar on the page.