When it comes to musical instruments like the piano, sometimes it is easier to show someone how to do 
something than it is to tell them how to do it. This is especially true when it comes to the combined art and science of piano tuning. On the surface, piano tuning is simply the act of adjusting the string tensions inside the piano to properly align the intervals between their tones so that overall, the piano is in tune with itself. However, since fine tuning a piano requires a measurement of the interaction between the notes, which can be different for every piano, in most systems of piano tuning, every pitch is actually measured and derived from its relationship to a chosen fixed pitch, which, in the case of modern musical instruments including the traditional piano and it’s more modern, electronic keyboard cousins, the standard chosen fixed pitch is usually A440 these days.
something than it is to tell them how to do it. This is especially true when it comes to the combined art and science of piano tuning. On the surface, piano tuning is simply the act of adjusting the string tensions inside the piano to properly align the intervals between their tones so that overall, the piano is in tune with itself. However, since fine tuning a piano requires a measurement of the interaction between the notes, which can be different for every piano, in most systems of piano tuning, every pitch is actually measured and derived from its relationship to a chosen fixed pitch, which, in the case of modern musical instruments including the traditional piano and it’s more modern, electronic keyboard cousins, the standard chosen fixed pitch is usually A440 these days.This means pianos require a different pitch from a theoretical standard and they are usually tuned to a modified version of the system called equal temperament derived from the relationship to the chosen A440 fixed pitch. The result is that accurate piano tuning can be a fairly complicated task, and most piano tuning today is done by professional independent piano technicians, piano re-builders, piano store technical personnel, and the occasional non-professional, but intrepid home piano hobbyist as well. Professional piano tuning is a business of its own in many cases, with training and certification available from various musical organizations or established guilds like the Piano Technicians Guild. Although it is time consuming, and can be a fairly costly process, most piano manufacturers today now recommend that their pianos should be tuned up at least two times every year.
The piano tuning process is also complicated by the fact that most classical western written music is based on a deviation from the theoretical equal temperament because very little music is actually played within a single octave. A pianist constantly plays notes spread over three and four octaves, making it crucial that the mid and upper range of the treble notes are stretched out to cover and conform to the non-harmonic overtones found in the lower registers. This means a true professional piano tuner is a person who is trained to be aware of which octaves in an instrument need "more" or "less" stretching to make a workable compromise between tonal brilliance, intonation and the tuner’s personal awareness of the different gradations of tone. In short, piano tuning is a process of tonal compromise that is best performed by a person with a good “ear” for the tones and enough experience to do the job correctly and repeatedly. Pianos are beautiful instruments that can make beautiful music, but they also must be correctly tuned or they won’t make music at all.