Why Does a Piano Goes Out of Tune ?

 Piano strings are under a great deal of tension that is supported by the frame, plate, pinblock, tuning pins, bridges and soundboard. Anything that affects the position of any of these parts will cause a change in tension and make the piano go out of tune.         Playing :  The louder and more often you play a piano, the faster it goes out of tune by a small amount. This is due to equalization of tension along the length of the strings. The better you "set the strings" during tuning, less this happens. (Arthur A. Reblitz)