Why does a piano go out of tune? The biggest reason besides structural issues involves the sound board. The sound board is about 3/8" inches thick and is made of wood. The wood expands and contracts with humidity fluctuations. The soundboard has a slight curvature also known as "crown". When the humidity increases, the curvature steepens and forces the strings to tighten and go sharp. The reverse happens when the humidity drops. Because the strings in a piano have varied lengths and tensions and are resting via the bridge on different locations on the sound board, the pitch moves unevenly. This process occurs whether or not the piano is played. Regular tunings and humidy control make for a more stable pitch situation across the piano's scale.
Tuning - Tuning is the process by which the notes are tuned in harmonic congruence. Except for very rare occasions, piano technicians use Equal Temperment which evenly spaces the musical intervals. This is done so the piano can sound harmonious in all keys. This is achieved by making certain intervals slightly sharp or flat from their pure ratios.
Pitch Correction - This is the process that puts the piano at the proper tension where A4 (note 49) is at 440 cycles per second. For most purposes, this is standard or "concert" pitch. While tuning involves putting the piano in harmony with itself, it may be at a different pitch. Humidity changes and neglect can affect the piano's pitch. This is a separate procedure from tuning.
Regulation - A piano has mechanical components that enable one to control its dynamic range. The principle ones consist of the key sticks, the hammers, the jacks (for escapement), the whippens, the dampers, and the connecting pieces called flanges. This group is known as the action. It is basically a lever system that requires the components to possess a proper geometry. This geometry is affected by wear and humidty fluctuations. Regulation involves adjusting action to restore its proper geometry and even touch.
Voicing - The hammers play a big part in a piano's voice. Their shape, weight, size, and density shape its tonal color. Harder hammers will produce a more bright tone and softer hammers will produce a more mellow tone. With use and wear, hammers become misshapen, more strident and the overall tone becomes uneven. Voicing generally involves softening, hardening, and sometimes reshaping them.
Repairs - Because there are many moving and structural parts of a piano, a piano occasionally may need a part repaired or replaced. Most general repairs can be done on site. If the piano is very worn or old, it may need more extensive work. If this is the case, a technician can refer the client to a rebuilder who is set up to do the work in a shop.
Humidity Control - Because much of a piano's parts consist of wood, they are affected by humidty swings. The tuning can move dramatically and the action may not work well with wide humidity swings. Often, a proprietary piano climate control sytem can stablize or mitigate damaging humidity changes.