Over time the piano will fall out of tune. In order to bring the piano back to concert pitch (A-440Hz) it is necessary to perform a pitch raise coupled with an overpull. A pitch raise is a gross adjustment of all the strings on the piano in order to bring the whole instrument up to the proper tension in order to proceed with a fine tuning. Typically, a pitch raise is coupled with an overpull because once brought up to the proper tension the strings will fall back approximately 25%-33%. The piano will not hold a tuning unless the pitch raise and overpull are performed.
For additional information regarding pitch raising please visit this link to the Piano Technicians Guild’s website: Pitch Raising