The reshaping of a tape recorder head after it has been worn. Tape heads need to have a carefully shaped surface where they come into contact with the tape for optimal performance. The process of dragging recording tape across the soft metal of a tape head for years causes it to wear down and develop a flat spot, which prevents the tape from being able to make consistent contact across the needed area. The result is degraded recording and/or playback quality, though since this is a very gradual process it is often not noticed by inexperienced users. Heads can be restored to original performance by relapping, which is essentially a very fine grain sanding or polishing process. Through this delicate process parts of a tape head are ground down (removed) to leave a properly shaped head; therefore it should only be done only by trained or seasoned head technicians. Generally heads can only be relapped one time – after that too much material is gone to be able to restore the proper shape without changing the length of the head gap, which would cause numerous problems.