Having observed that the piano tuner turns tuning pins ("pegs"), most people conclude that pianos go out of tune because tuning pins turn on their own. Although that is true in pianos with a worn or cracked pinblock (slab of laminated hardwood that holds tuning pins in place), tuning pins don't turn spontaneously in any piano that can be tuned normally. If turning the tuning pin counterclockwise requires effort, the pin will not move unless you hit it with a hammer or drop the piano.
Instead, the tuning is affected mainly by:
The soundboard is made of spruce, which responds to changes in humidity by swelling and shrinking across the grain (perpendicular to the long bridge). Because wooden slats, called ribs, are glued to its underside, the soundboard can't expand in width so it curves upward, forming a "crown" that resists the downward pressure of strings. When the soundboard swells during humid months, its pressure against the strings increases more in the middle than in the bass and treble, causing a corresponding rise in pitch of the tenor and middle sections. Conversely, shrinkage during the heating season causes a drop in pitch, also disproportionately more in the middle.
Even the unison tuning is affected by humidity. Because the swelling and shrinking of the soundboard changes the angle at which the strings cross the bridges, and—looked from above—the long bridge is not perpendicular to the strings, unisons go out of tune as well, with the right string in each unison affected the most, the middle one slightly less, and left one the least.
The only solution is to maintain the soundboard at steady humidity levels. Some people believe that old soundboards stop responding to humidity changes. Whereas it is true that a soundboard that has lost most of its compression will react less to humidity swings, it will react. Humidity control is still relevant.
With time, even in stable climate conditions, the soundboard crowning decreases and the pitch drops, but those changes occur slowly and can be easily compensated during regular tunings.
Controlling humidity is a good idea to protect other wooden parts, such as the keys, action parts, and the pinblock, from loosening, warping, cracking, or delaminating. More on pianos and humidity...
Sudden changes of more than 10 degrees F (5 degrees C) make the strings contract or loosen enough to cause a change in pitch. A rush of cool air from air conditioning vents can make the pitch rise a few cents within minutes. Likewise, warm air from heating ducts or sun hitting the piano will make the pitch drop. The change is not evenly distributed and the piano is perceived as going out of tune. Fortunately, if the temperature change is minor and short-lived, the piano will go back in tune on its own. However, repeated changes—for example from constant cycling of the climate control system or daily warming in a sun-lit room—can cause not only the tuning to go out, but the soundboard and pinblock may crack and the veneer may start coming off.
Tuning pins are cylinders made of resilient steel, and they bend and twist during tuning. They are held tightly by a wooden pinblock approximately 3/8" (10 mm) below the surface of the cast iron plate, but the string is wrapped around the pin significantly higher, above the plate, allowing the pin's bending and twisting to affect the pitch of the corresponding string. When the tuner wants to pull up the pitch slightly, he or she has to overshoot the target pitch in order to turn the pin minutely in the pinblock, and then has to release the extra bending and twisting of the pin, hopefully reaching the desired pitch. Because the target pitch is rarely reached on the first try, the tuner repeats this process several times while pounding the key to help the string overcome friction and slip across its various bearing points.
The tuning pin torque plays a significant role in this process. If making a correction of 1 cent (1/100th of a half step) requires overshooting the pitch by 30 cents—an amount quite common in new pianos—it becomes clear why it takes a great amount of skill and experience to release the bending and twisting of the tuning pin just the right amount. Add to that the uncertainty of string friction at bearing points, and you realize that a stable tuning is the result of a tuner's highly intuitive sense of tuning pin and string behaviors. A smallest misjudgment will cause the string to slip up or down during loud playing or if the temperature changes a little.
40,000 pounds (18 tons) of string tension is held by a cast-iron plate, which is affixed to a strong wooden structure typically comprised of a laminated (or composite) rim and one or more beams. On the side of the tuning pins, the tension is transferred to the plate by the pinblock, a slab of laminated hardwood. The contact between the pinblock and plate must be extremely solid to prevent inadvertent movement of the pinblock, which would destabilize the tuning. If the plate itself is cracked, not fastened solidly to the rim, or not well supported by the nose bolts in the middle, its own movement can cause tuning instability. Finally, the frame and case must be solid and disallow any movement.
In older upright pianos the case can tilt slightly, causing the piano to go out of tune when it is moved across an uneven floor. That's why it is recommended to tune an upright piano in the exact position where it will remain after the tuning.