Piano tuning is the art of making minute adjustments to the tensions of the strings of a piano to properly align the intervals between their tones so that the instrument is in tune will all the intervals on the piano. The meaning of the term in tune in the context of piano tuning is not simply a particular fixed set of pitches. Fine piano tuning requires an assessment of the interaction among notes, which is different for every piano because of the different lengths of string and the wooden bridges, thus in practice requiring slightly different pitches from any theoretical standard. Pianos are usually tuned to a modified version of the system called equal temperament. In all systems of tuning, every pitch may be derived from its relationship to a chosen fixed pitch, which is usually A440. Concert pitch Many piano manufacturers recommend that pianos be tuned twice a year.