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 (sharps or 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.
