Maintaining these instruments so they perform properly is the job of musical instrument repairers and tuners.
Piano tuners adjust piano strings to the proper pitch.
A string's pitch is the frequency at which it vibrates and produces sound when it is struck by one of the piano's wooden hammers. Tuners first adjust the pitch of the A string. Striking the key, the tuner compares the string's pitch with that of a tuning fork. The pitch of each of the other strings is set in relation to the A string. The standard 88-key piano has 230 strings and can be tuned in about an hour and a half.