Time signatures (not to be confused with key signatures!) are the two numbers at the beginning of a piece of music. The time signature describes the pulse of the music, giving strong and weak beats. These strong and weak are separated into groups by bar lines in written music. Each bar can only contain a specific number of beats, and we find this number by looking at the time signature.
What’s in a time signature?
A time signature is made up of two numbers:
Top number= number of beats per bar
Bottom number= type of beats in a bar
The time signature usually stays the same throughout a piece of music, and so all we need is a single time signature at the beginning of the piece. However, sometimes it changes, in which case there will be a new time signature written at the beginning of the first bar that contains the new pulse. This is shown in the illustration below, with the first two bars in 4/4, and the last two bars in 2/4.