The individual beats in a piece of music are divided into measures, also known as bars, which are repeating groups of beats. And a time signature is made up of two numbers that appear above and below each other, as in these examples:
, etc., which can also be represented as 4/4 and 3/4
These numbers appear in the first measure of a chart or score to tell you how to count the piece of music. The top number represents the number of beats in each measure, and the bottom number tells you what kind of note gets one beat. For example, in the second example above the “3” on top means there are three beats per measure, and the “4” on the bottom means that a quarter note gets one beat. Of these two numbers, the one that matters most in determining the feel of a piece of music is the top one — the number of beats per measure.
The most common time signature is 4/4. In this time there are four beats per measure, and the quarter note gets one beat. When the bandleader counts the song off with a “1-2-3-4” he is counting off one measure at the exact speed that you'll be playing the song.
There's virtually no limit to the number of possible time signatures. You can have as many beats as you want in a measure — a whole piece of music could be written as one measure if you wanted to go to extremes.
Each time signature has its own emotional feel. Besides 4/4 the most common time signatures and their feels are:
2/4 which has a hopping polka feel, counted 1-2, 1-2
3/4 which has a circular or waltz feel, counted 1-2-3, 1-2-3
5/4 which has a measure-of-three with a two-beat accent feel, counted 1-2-3-4-5, 1-2-3-4-5
7/4 which has a measure-of-four with a three-beat accent feel, counted 1-2-3-4-5-6-7, 1-2-3-4-5-6-7
1/4 which has a three-measures-of-three with a two-beat accent feel, counted 1-2-3-4-5-6-7-8-9-10-11, 1-2-3-4-5-6-7-8-9-10-11
But the fact is, you can create any time signature you want, assigning any number of beats to the measure and any note value as getting one beat.