拍子記号
Numbers written at the start of [[sheet music]] indicating the number of beats in a bar. These are usually two numbers written one above the other after the key signature ([[sharp sign|sharps]] or [[flat sign|flats]] at the start of the line), although other varieties such as time symbols and mixed or fractional signatures do exist, and not all music has a time signature.

If the number of beats in a bar changes at any point in the music, there will be a new time signature at that point, unless there was no time signature or a mixed signature at the start.
time signature