The back check is screwed to the key, and as the hammer head rests against it after striking, the use of the contrivance called the back catch in the upright is unnecessary.

[Illustration:  ACTION OF THE SQUARE PIANO.]

     ACTION OF THE SQUARE PIANO.

    A. Action Frame.

    B’s Indicate the Cushions, or Bushing, of felt, cloth or leather.

    C. Balance Rail.

    D. Balance Pin.  Round.

    E. Mortised Cap for Balance Pin.  Bushed.

    F. Key.

    G. Lead.

    H. Back Check.

    I. Bottom or Key Rocker.

    J. Bottom Screws; used to regulate height of Jack.

    K. Jack.

    L. Jack Spring; concealed under Bottom.

    M. Center Pin to Jack.

    N. Hammer Rail.

    O. Regulating Screw.

    P. Regulating Button.

    Q. Flange Rail.

    R. Flange.  Split.

    S. Flange Rail Screw.

    T. Flange Screw, to regulate jaws of flange.

    U. Hammer Butt.

    V. Center Pin.

    W. Hammer Stem or Shank.

    X. Hammer Head.

    Y. Hammer Felt.  Treble hammers sometimes capped with buckskin in
    old instruments.

     TOP ACTION OF SQUARE PIANO.

    1.  Damper Lifter Wire.

    2.  Damper Lifter Buttons.

    3.  Damper Felt.

    4.  Damper Head.

    5.  Damper Lever.

    6.  Damper Leads.

    7.  Shade, supported by wire stanchions, on top of which are
    screwed shade buttons.

    8.  Damper Rail.  Tilted by Loud Pedal Rod which raises all the
    dampers simultaneously.

    9.  Damper Flange.

    10.  Flange Screw.

    11.  Damper Lever Center Pin.

     THE TRAP ACTION

     consists of Pedals, Pedal Braces, Pedal Feet, Pedal Rods, Roller
     Boards or Elbows, Studs, Plugs, Trap Springs, Wires and Lifter
     Rods.

     The cut is from the French action.  Nearly all square pianos in
     use at the present time are of this type.

The hammer rail in the square, in addition to serving its purpose as a rest for the hammers, also serves the purpose of the regulating rail, as you will see the regulating screw, with its button, attached to it.  This rail is stationary in the square, not moving toward the strings and shortening the stroke as it does in the upright when the soft pedal is used.  The soft pedal in the square piano simply interposes a piece of felt between each hammer and its corresponding string or strings.  This felt being much softer than that of the hammers, the tone is greatly subdued.

The mechanical arrangement of the dampers is very different in the square from that in the upright.  The dampers are above the strings.  Instead of springs to hold them against the strings, they simply rest upon them with their weight.  In many old squares some of the dampers fall upon nodal points, causing defective damping or harmonic after-tones.