To produce the correct pitch, the string must be of the right tension, which is brought about by winding one end of the string around the tuning pin until the proper degree of tension is reached.  This must be decided by the ear of the tuner.  Two strings of equal thickness and equal length produce the same tone when brought to the same tension; the result being known as “unison.”  A defect in the unison being the easiest way in which to detect the beats, we advise that the student practice on it first.

After taking out the panel, the first thing to do is to place your rubber mute between two trios of strings (if the piano is an upright which usually has three strings to a note) so that only two strings sound when the key is struck.  Select some key near the middle of the keyboard.  Strike the key strongly and hold it down.  If the two sounding strings give forth a smooth, unwavering tone—­a tone that sounds as if it came from one string, the unison is perfect.  If you find it so, remove the mute and place it on the other side of the trio of strings.  If the piano has been tuned recently by an expert, you may have to continue your search over several keys before you find an imperfect unison; but you will rarely find a piano in such perfect tune that it will not contain some defective unisons.  However, if you do not succeed in finding a defective unison, select a key near the middle of the key-board, place your mute so that but two strings sound, and with your tuning hammer loosen one of the strings very slightly.  Now you will notice a throbbing, beating sound, very unlike the tone produced when the strings were in exact unison.  See if you can count the beats.  If you have lowered the tension too much, the beats will be too rapid to permit counting.  Now with a steady and gradual pull, with the heel of the hand against some stationary part, bring the string up slowly.  You will notice these waves become slower and slower.  When they become quite slow, stop and count, or wave the hand in time with the pulsations.  After practicing this until you are sure your ear has become accustomed to the beats and will recognize them again, you may proceed to perfect the unison.  Bring the string up gradually as before, and when the unison is reached you will hear one single, simple, musical tone, as though it were from a single string.  Never have more than two strings sounding at once.  You might go over the entire key-board now and correct all the unisons if the scale is yet fairly good.  See which string is, in your opinion, the nearest to correctness with respect to the scale, and tune the other one, or two, as the case may be, to it.  If the scale is badly out of symmetry, you will not get very good results without setting a temperament; but the tones will sound better individually.  This experiment is more for practice than for improving the piano.

The cause of the waves in a defective unison is the alternate recurring of the periods when the condensations and rarefactions correspond in the two strings and then antagonize.  This is known in physics as “interference of sound-waves.”