Guitars, violins, cellos, and other instruments with strings or wires, are normally tuned before each use, and checked regularly, usually by the player. In an ideal world, this would also apply to the piano. I like to make the point that concert instruments are often tuned just as frequently. However, piano tuning is the job of a skilled specialist, so it is impractical and expensive to have it tuned all the time. Conventional advice is for piano tuning to be carried out twice (or at least once) a year.
Pianos go out of tune due to a variety of factors, the main ones being (1) directly from being played: all other things being equal, the more a piano is played, the more out of tune it will go; and (2) changes in temperature and humidity: cause the soundboard to expand and contract. The soundboard, as its name suggests, is a board (spruce) to which the strings are indirectly connected, and is slightly convex in shape. Expansion and contraction of the piano’s soundboard alters the tension on the strings, and, consequently, the tuning. These changes occur seasonally—summer to winter, and vice versa—and it is for this reason that tuning twice a year is recommended.
Concert pitch (or standard pitch) is the pitch to which musical instruments are tuned, so they sound in tune when played together. It has changed throughout history, and varies slightly in different parts of the world. British Standard Pitch is A440, i.e. the A in the middle of the piano keyboard is set at 440 Hz. Instruments all tune to this standard, so, therefore, an A on the piano should sound the same as an A on a violin, guitar, and flute, etc.
Pianos are tuned by first setting a note in the middle of the keyboard to a tuning fork. This is usually either A or C. From this benchmark, the next note is tuned. Then, from that one, the next etc., so each note is tuned in sequence from notes that have been tuned. Once the first note is set, the next step is to tune the ‘equally tempered scale’, the technicalities of which are not really important, except that the result is a chromatic octave of notes all in tune, in the middle of the piano keyboard. For example, if tuning from C, the scale would be the octave from F below to F above middle C. This ‘scale’ consists of twelve equally divided semitones, and allows music to be played in any key. The rest of the piano is then tuned in octaves, up the treble, and down the bass, until completed.
Pitch and Stability
If piano tuning is neglected, the piano can go flat. After long periods, it is not unusual for the pitch to drop by a semitone or more. It may then be necessary to correct this by bringing the piano up to pitch, provided the strings are in good enough condition to take the extra tension. The further a note has to be altered to tune it, the more it will respond by going out again. Resolving this problem when raising the pitch is achieved by going through the tuning more than once (within the same visit), depending on how flat the piano is. Usually two or three times is enough, and on each run through, the stability of the tuning improves. It is to be expected that following a pitch raise, the tuning will be less stable than normal, and a further tuning within six months is recommended.