Pitch Correction/Pitch Raising Explanation
Pitch raising/lowering/stabilizing is the basic "foundation" for any truly "fine" tuning. Without laying this foundation, no "fine" tuning will be possible as a result.
Extraordinary Counter Measures are required (must be employed) whenever the pitch deviates more than +/- 3 cents from A-440 HZ (One "cent" equals 1/100th of a semitone, or a half-step). This seems like an insignificant amount, however, in terms of a piano's pitch stability, it is a crucial consideration. Precise tuning requires accuracies exceeding 0.10 to 0.01 cents (1/1000th to 1/10,000th of a semitone).
Pitch raising/stabilizing involves a process of compensation for the piano's soundboard compression, as more pressure is applied to it through the change in string tension.
Typically, the pitch will drop approximately 25-35% of the amount it is raised on the treble bridge(s), and approximately 10% on the bass bridge. The pitch raising process, referred to as "overstretching" or “overpull,” compensates for this "phenomenon." When properly executed, the pitch will "set" and "stabilize" normally as if it had never been raised.
The complete process takes three (3) "passes" (tunings)... the first two "raise" and "stabilize" the pitch to within less than one cent (1¢)... the final one is when the actual "Fine Tuning” occurs, and the pitch "set" is achieved.
Now, after all this is said and done, a "local" climate stability is required to achieve a long lasting tuning stability. Daily (even hourly) humidity fluctuations cause the soundboard's "crown" to rise (increase) & fall (decrease)/expand & contract, which correspondingly, also causes the pitch to rise and fall. Eventually, depending on the frequency, duration, and severity of the fluctuations, tuning will again be required. There are no guarantees in predicting the weather, or in the tuning requirements of a piano (except that it will be needed again). A piano's tuning is probably over 100 times more stable than a harpsichord's... the latter doesn't really endure more than 24 hours.
Ideally, a piano should be tuned, if not just retouched, before every performance event.