Tuning a Piano
Unisons
There are 88 notes on a piano, but over 200 strings! This is because in the top two-thirds or so of the piano, their are three strings per note. In the top half of the bass section, there are two strings per note. Each group of two or three strings should be vibrating at the same frequency as its neighbor(s). It should sound like one note! These are called unisons.
If the unisons are "out," the note will sound "fuzzy" and distorted because the sound waves coming from the two or three strings are close to the same pitch, but not the same pitch. When the unisons are perfectly in tune with each other, the note will sound "pure" or "clean."
There are several techniques which qualified piano technicians use to make sure the unisons not only get in tune with each other, but stay in tune for as long as possible after the technician has left. The technician learns to use the tuning hammer in such a way as to ensure that the tuning pin will be in a stable position after the tuning hammer is removed from the end of the pin. The technician should also give "test blows" to each note as s/he tunes it. This means striking the note hard in order to equalize the tension on different parts of the string. If a tuner tunes quietly, the tuning cannot be stable. The first person to sit down at the piano and play it forcefully will knock it out of tune.