Why do pianos go out of tune?

Piano strings change pitch for two primary reasons: the initial stretching and settling of strings when the piano is new, and soundboard movement due to humidity, temperature, and barometric-pressure variation.

With new pianos, the pitch drops quickly for the first couple of years as the new strings stretch and wood parts settle.  So it’s very important to maintain any new piano at the proper pitch during this period.  (Most piano manufacturers recommend three to four tunings the first year, and at least two per year after that.)

As for atmospheric changes:  When the relative humidity goes up, the wooden soundboard swells, increasing its crowned shape and stretching the piano’s strings to a higher pitch, especially in the middle octaves.  When the humidity goes down, the soundboard flattens out, lowering tension on the strings and causing the pitch to drop.  The dry-season drop tends to exceed the wet-season rise, so the net result is a drop in pitch each year the piano isn’t serviced.

Your piano was designed to sound its best when tuned to the international pitch standard, A–440 (the A above middle C, which vibrates at 440 cycles per second).

But many older pianos have aged to the point where it’s dangerous to try bring them back up to that standard pitch.  It risks straining the pegs, warping the soundboard, and stretching the strings — even to the breaking point.

Unless it’s absolutely necessary for your older piano to accompany other instruments that can’t easily be tuned to match it, it’s usually best to keep it tuned to the lower frequencies it’s become acclimatized to.