Piano tuning is the act of making adjustments to the tensions of the strings of a piano to properly align the intervals between their tones so that the instrument is in tune.
Many factors cause pianos to go out of tune, particularly atmospheric changes. Humidity will affect the pitch of a piano; high humidity causes the soundboard to swell, stretching the strings and causing the pitch to go sharp, while low humidity has the opposite effect. Changes in temperature can also affect the overall pitch of a piano.
Under normal circumstances, the frequency to tune a new piano should be tuned four times the first year and twice a year subsequently. If no one is playing, it should be tuned at least once a year to keep it from damage.