So far, we have only looked at measures in 4/4, meaning each measure has four quarter notes. We can also have measures in 3/4, meaning each measure has three quarter notes. Each of these ratios is known as a "time signature", or "meter". It is possible to have any positive integer in the numerator of a time signature: so 1/4, 2/4, 3/4, 4/4, 5/4, and so on, are all valid time signatures. However, in the denominator of a time signature, you may have only a power of two: so 2/2, 5/8, and 17/16 are valid time signatures, but 4/3, 2/7, and 8/5 are not.
One remaining aspect of rhythm is speed. Sometimes your beat is fast; other times it's slow. The speed of the beat is known as "tempo". Most musicians are used to representing tempo as beats per minute, or BPM. This means that if your beat is a quarter note—as in 4/4 or 3/4—a tempo of 60 BPM means there are 60 quarter notes per minute, or one quarter note every second. If your beat is an eighth note, as in 6/8 or 12/8, a tempo of 60 BPM means there are 60 eighth notes per minute, which means there are 30 quarter notes per minute, or one quarter note every two seconds. Musicians often omit the letters "BPM" when discussing tempo, instead saying things like "quarter note equals 90" or "eighth note equals one-hundred thirty-two".