The purpose of the tuning hammer is that of altering the tension.  The purpose of striking the string by means of the key is twofold:  first, to ascertain the pitch of the string, and second, to equalize the tension of the string over its entire length.  Consider the string in its three sections, viz.:  lower dead end (from hitch pin to lower bridge), vibrating section (section between the bridges), and upper dead end (from upper bridge to tuning pin).

When placing the hammer on the tuning pin and turning to the right, it is evident that the increased tension will be manifest first in the upper dead end.  In pianos having agraffes or upper bridges with a tightly screwed bearing bar which makes the strings draw very hard through the bridge, some considerable tension may be produced in the upper dead end before the string will draw through the bridge and increase the tension in the vibrating middle.  In other pianos the strings “render” very easily over the upper bridge, and the slightest turn of the hammer manifests an alteration of pitch in the vibrating section.  As a rule, strings “render” much more easily through the upper, than the lower bridge.  There are two reasons for this:  One is, that the construction of the lower bridge is such as to cause a tendency in this direction, having two bridge-pins which stand out of line with the string and bear against it in opposite directions; the other is that the lower bridge is so much farther from the point where the hammer strikes the string that its vibration does not help it through as it does at the upper bridge.

Now, the thing desired is to have the tension equally distributed over the entire length of the string.  Tension should be the same in the three different sections.  This is of paramount importance.  If this condition does not obtain, the piano will not stand in tune.  Yet, this is not the only item of importance.  The tuning pin must be properly “set,” as tuners term it.

By “setting the pins,” we mean, leaving it so balanced with respect to the pull of the string that it will neither yield to the pull of the string nor tend to draw it tighter.  Coming now to the exact manipulation of the tuning hammer, there are some important items to consider.[F] Now, if the tuning hammer is placed upon the tuning pin with the handle straight upward, and it is pulled backward (from the tuner) just a little, before it is turned to the right, the tension will be increased somewhat before the pin is turned, as this motion, slight as it may seem, pulls the pin upward enough to draw the string through the upper bridge an infinitesimally small distance, but enough to be perceptible to the ear.  Now if the hammer were removed, the tendency of the pin would be to yield to the pull of the string; but if the pin is turned enough to take up such amount of string as was pulled through the bridge, and, as it is turned, is allowed to yield downward toward the pull of the string, it will resume its balance and the string will stand at that pitch, provided it has been “rendered” properly over the bridges.