A piano may look like a solid instrument that requires no tuning, but this isn't the case. In fact it is recommended that a new piano be tuned three or four times during its first year, and twice yearly thereafter whether it is played or not. But how does a piano get out of tune in the first place, and why does such mystery surround piano tuning?
The answer to both questions is in the construction of the piano. Pianos are made of materials that age, "breathe" and change shape with time and weather conditions. Wood, iron, leather, felt, and steel all go into a piano. Each piano has thousands of intricate moving parts and a complex system of 250 or more high-tensile-strength steel strings, pulled across an iron harp that holds 40 tons of string tension! Each string is wound around a tuning pin that is driven into a wood pin block. The pins must fit very snug in order to hold the tension. In piano tuning, these pins must be carefully rotated one at a time to change the pitch of each accompanying string. This is done with a tuning hammer and requires a great amount of carefully applied strength.
But there's more to piano tuning than just turning a tuning peg. When you depress a single key on a piano, a hammer strikes a set of two strings for the lower octaves and three strings for the higher octaves. This set of strings is tuned to the same pitch to produce the single note. If one of the strings in the set is flat or sharp, the single note can be out of tune with itself! In piano tuning a single string within the set is sounded individually and rubber wedges are used to stop the vibrations of other strings. Once that string is tuned, a second string is allowed to vibrate along with the first and is tuned to it, and so forth and so on. It is an extremely slow, methodical process that requires expertise, a good ear and lots of patience.
Piano strings last about 20 years before they should be replaced. This is a costly expense so many people simply live with old strings. Piano tuning then becomes even more difficult because the older the strings gets past the 20-year mark, the more susceptible they become. The peg must be turned very carefully and the string allowed "to rest" between small adjustments. There will come a time, however, where tuning is no longer safe to perform without running the risk of breaking a string.
In the United States, piano tuning generally costs about $120 and is considered a regular necessity to keep a piano in good playing condition for the duration of its life. Do-it-yourself piano tuning is not recommended though books and guides are available on the subject for those interested.