Tremolo Arm, Guitar Body

It remains the only widely used design whose mechanism is entirely above the belly of the guitar body, making it the only design particularly suitable for acoustic and semi-acoustic guitars.

In the original design, this was based on the principle of the 'knife edge' balance. A bevel on the front underside of a steel top plate formed a very wide angle knife edge that rested on the top surface of the guitar body. A small imbalance in tension between the pull of the strings and the counterbalancing pull of the tremolo springs held the pivot edge firmly in place against the body. Six hardened steel woodscrews passing through slightly oversize holes just in front of the pivot point, stopped the bridge from being pulled towards the neck end of the guitar. The upper portion of the screws is smooth, not threaded. These six screws are often mistakenly assumed to be the pivot point rather than the hidden knife edge. This design works, in spite of the friction caused by the edges of the six holes sliding up and down the screw shafts when vibrato is applied.

The tailpiece consists of a solid block of metal, mounted behind the tremolo plate and secured to it by three machine screws, and passing right through the guitar body. In a chamber routed into the back of the guitar are up to five (normally three) long coil springs which connect to the back of the tailpiece block, and whose tension balances that of the strings. The tremolo arm also passes through the tremolo plate and tailpiece block, providing direct and rigid connection. Ignoring the bridge adjustments, this mechanism has only two moving parts, one of them the arm itself, the same as the Bigsby. But unlike the Bigsby, the synchronized tremolo moves the bridge as well as the tailpiece, varying both the length and tension of the strings.

However, in the Telecaster the ferrule end is held by a collar firmly anchored to the guitar body; In the Stratocaster, it is held by the moving metal block through which the strings pass.

Source: Wikipedia > Tremolo Arm