If a piano has gone without tuning for an extended period, its pitch may have dropped far below A440. This means that each of its approximately 220 strings needs to be tightened considerably, adding tremendous additional tension to the piano's structure. The problem is that as each string is tightened, the additional load causes the pitch of previously adjusted strings to change. Thus it is impossible to make a substantial change in pitch and end up with fine, accurate tuning in one step. Instead, a process called "pitch raising" must first be done, in which all strings are raised to their correct average tension levels. (Likewise, when a piano's pitch is higher than standard, a pitch lowering procedure must be done to reduce string tensions to approximately correct levels.) Only then can the piano be accurately tuned. In other words, accurate tuning is only possible when all strings are so close to their proper tension that only small further changes are needed during tuning. These small changes then do not disturb the tuning of other strings.
Get detail info: