In most music, beats are grouped into bars. Bars (also called measures) are important for the dance (physical or mental) that the listener experiences. The most common grouping in western music is four. You may have noticed that in most of the exercises in chapter three, there was a vertical line every four beats. Those vertical lines are called bar lines and are used to group the beats into bars. Grouping beats into four creates the time signature of four-four. The time signature is used to describe the way that the beats are grouped.
In sheet music, time signatures appear at the beginning of a song and are represented by two numbers stacked on top of each other. In four-four, the first “four” (or the “4” that’s on top) says that there are four beats in a bar, and the second “four” (the bottom “4”) says that the quarter note is the beat. So in four-four, there are four quarter notes in a bar. In three-four, there are three quarter notes in a bar. In six-eight, there are six 8th notes in a bar. Below are examples of these time signatures.