Your piano is designed to sound its best when tuned to A-440 (A above middle C vibrates at 440 cycles per second), the international pitch standard. At this pitch, power and tonal range are optimum and your piano will match the pitch of other instruments. When your piano varies from A-440, pitch adjustments are required to bring it back to standard. By always maintaining your piano at standard pitch, you create long-term tuning stability because the strings and structure stay in equilibrium. You also ensure proper ear training because you always hear your music in the correct key.
If a piano has gone without tuning for an extended period, its pitch may have dropped far below A- 440. 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 a 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.