rather than a "vibrato arm", conversely referring to the tremolo circuit on his amplifiers as "vibrato". Vibrato systems send a guitar out of tune when friction inhibits the vibrating length of string from returning to its original tension after a pitch bend. Fender's design is 'synchronized' in the way that the bridge saddles and string ends move together as one rigid unit, mostly eliminating sliding between string and saddle. The unit attaches to the guitar's body with six steel wood screws. So the bridge can pivot smoothly about the screws, the upper portion of each one is unthreaded, they are not tightened all the way, and they pass through slightly oversized holes in the plate at the center of the design. Six bridge saddles are held against this plate by string tension, individually adjustable both for height and intonation. Another of the components which make up the tailpiece consists of a solid block of metal, commonly made of zinc, but aftermarket products can be made out of materials such as titanium or brass (often like the saddles). The different compositions, as well as its mass, helps to determine the quality of the sound of the guitar. The block is secured to the base plate by three machine screws and resides in a cavity routed all the way through the guitar's body. In this cavity, up to five coil springs tether the end of the bridge block to the body, counteracting the pull of the strings, should this be the setup choice for the guitarist. There can even be differing sizes of tremolo arms in thickness depending on the date and country of manufacture. The number and length of springs may be adjusted to set the neutral position of the bridge, determining the range of upward and downward pitch bending available. A bridge set to 'float' off the guitar lets the guitarist raise the pitch until the bridge presses against the body. Pitch bends are accomplished by pulling up or down on an arm screwed into the tailpiece block, usually free to swing. The Fender synchronized tremolo is the most widely copied vibrato system. The original design is still in production virtually unchanged today. The synchronized tremolo may have been the reason for the popularity of the Stratocaster among rock musicians in the late '60s and '70s. Owing to its superiority in aggressive use, all Fender guitars using any other vibrato