Why do pianos go out of tune?

Did you know that pianos go out of tune even if no one plays them? The major reason that pianos go out of tune is the change in humidity of their surroundings. You’ll often find that a piano sounds great one day and then the weather changes and suddenly it sounds different. Besides having it tuned on a regular basis, it is good to keep the piano in a heated and air conditioned space, away from drafts. This also helps keep rust off the strings and other metallic parts.