How can you use a tuning fork 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 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. Once the "A" key is in tune, you would then adjust all of the instrument's 87 other keys to match.

Explanation:

Most tuning forks are made to vibrate at 440 Hz, a tone known to musicians as "concert A."