The strings are under considerable tension and over time this tension will stretch them, lowering the pitch resulting in the piano sounding out of tune. Unfortunately, no matter how expertly a piano is tuned, atmospheric variations, particularly humidity and the nature of the piano's construction constantly conspire to bring it off pitch.
Our piano tuner will restore the pitch providing the piano is structurally sound and the strings are in good condition. For example a Steinway piano is tuned many times before it leaves the factory. It is tuned to and should be maintained at a 440 pitch for New York models or 443 pitch for Hamburg models. These are the internationally accepted standards and the standards for which all Steinway pianos are engineered.
Most pianos have been designed and built so that in normal use and under normal conditions it should need only periodic tuning. We recommend that your tuner should be called at least 3 or 4 times a year. You, however, are the final judge and should have the piano tuned as often as you think necessary.
To put the matter of tuning into perspective, remember that a concert piano is tuned before every performance and a piano in a professional recording studio, where it is in constant use, is tuned 3 or 4 times each week as a matter of course. Tuning is an art practiced by skilled professionals and under no circumstances should anyone other than a professional be allowed to try to tune your piano.