W, is the wippen.  Those pieces upon which or by which the small letter g is shown are the flanges.  The one at the left of the wippen is called the wippen flange.  It is made fast to the main rail by a screw, and upon it the wippen is hinged by means of a “center-pin” at the lower end.  The center-pin in the wippen is driven through a hole in which it fits tightly and immovably in the middle part, and it (the center-pin) is consequently stationary in the wippen.  The flange extends down at the sides of the wippen and the holes in flange are made large enough to receive bushing cloth in which the center-pin works freely but not loosely.  All flange joints are of this nature; some, however, are provided with a means for tightening the center-pin in the middle portion of the joint.

j, is the jack.  The purpose of the jack is to communicate the motion of the wippen to the hammer.  The precise adjustment of the jack and the adjacent parts upon which it depends for its exact movements, play an important part in regulating the “touch” of the piano, and will be fully entered into in following lessons.

js, jack spring.  Its purpose is to hold the jack inward against the “nose” or “heel” of the hammer butt. (See Bt, hammer butt.)

Rr, regulating rail.  The regulating button is shown attached to the rail by the regulating screw which is turned by means of its ring on top of Rr.  The purpose of the regulating button is to throw the point of the jack out of the nose of the hammer butt, and allow the hammer to rebound from the string.  If the button is too high, it does not throw or trip the jack in time to prevent blocking.  When the button is too low, it disengages too soon, and much of the force of the key is lost before it reaches the hammer.

BR, is the block rail, felted on the side next to the jack which strikes against it when thrown from nose.  This rail is absent in some actions, in which case the back of the jack is felted and strikes against the “back catch,” which is also felted on inner side. (The back catch has no mark in the cut, but is explained below in connection with the “back check.”)

BC, is the back check which is simply a piece of wood with a thick piece of felt glued to the inner face and suspended on a wire.

BCW, back check wire supporting the back check, and screwed to the wippen.  The purpose of the back check is to check the hammer by coming in contact with the “back catch” (the backward projection of the butt), at a short distance from the string in its return, and prevent the hammer from falling entirely back to its rest position, thereby preventing quick repetition.