Why does my piano go out of tune?
For a piano that is in good condition, the main reason for tuning decay is the effect of relative humidity on wood. As the wooden components of your piano gain or lose moisture with the climatic change of seasons, these parts ever-so-slightly change size. It's the same reason that a door in your home may stick in the summer, but swing freely in the winter. No part of the piano is affected more by this change than the soundboard, and the soundboard is bearing an indirect "downward" force from the strings. When the soundboard looses moisture, its crown (or curved profile) relaxes, decreasing the string tension, and the piano, especially in the middle range, falls flat. When moisture is gained in the summer, the piano goes sharp for the opposite reason. Controlling the seasonal fluctuation will greatly improve tuning stability.
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