As time and other factors act on a piano, the tension on the strings tends to drop until the pitch is very low. If a tuner brings the strings to the correct frequency in one pass (A440 hertz), he or she will find that no matter how carefully they try to do it, the increasing tension created by the tuning process will change the strings they first tuned, often significantly.
THERFORE: If a piano is too far out of tune, it will not stay in tune after one “tune.” It will require a preliminary “rough” tuning first, known as a pitch raise. The internationally established correct frequency for the note “A” above middle “C”, is A440 and is the starting pitch from which the rest of the piano is tuned, theoretically, a piano can be in tune with itself even if "A" is far from the "correct" frequency.
A=440 hertz or CPS (Cycles per second) relates to the speed that “Note A” beats to attain the correct pitch.
Pianos that have not been tuned in two years may require this extra work, but new instruments where the strings are still stretching can require this process as well. Sometimes age and the pianos condition have deteriorated so far as to make retuning the piano to concert pitch impossible. This is a discussion that needs to be held between the tuner and the customer as a experienced tuner can fairly quickly determine if a piano can be “A” tuned or “B” tuned to any sort of standard that would be acceptable to both the customer and the tuner.