Beats
Beats occur when 2 waves with similar but not identical frequencies are added together.
The longer the beats are the closer they are to the same frequency.
Beats can be used to tune a piano. To tune a piano, you would start by playing the piano's "A" key while ringing an "A" tuning fork. If the piano is out of tune you'll hear a distinct warble (these are beats) between the note you're playing and the note played by the tuning fork; the further apart the warbles, the more out-of-tune the piano. By either tightening or loosening the piano's strings, you reduce the warble until it's in line with the tuning fork.