practice, this subsides. String gauge also plays a big role; typically thicker strings are more difficult to bend. Notable guitarists who used very heavy-gauge strings while still producing musical bends include Stevie Ray Vaughan and Peter Green. Bending the thinner strings can make them cut into the fingers of a beginner guitar player. Over time, guitar players who play regularly develop callouses on their fingertips, which develop into hardened skin upon healing. Behind-the-nut pitch-shifting Also known as "behind-the-nut bending" Pressing the string between the nut and the machine head (tuning key), causes the pitch to shift. Examples include Jimmy Page's unaccompanied solo break in Led Zeppelin's "Heartbreaker". Classical guitar (nylon-string): This works on the unwound strings on a classical (nylon-string) guitar, and also works better on the strings whose heads (tuning keys) are further from the nut. Bass guitar: works on all strings The particular advantage of this technique is that unstopped notes can be pitch-shifted (bent). Variations Several strings can be bent at once. Innumerable bend patterns exist: for example, straight bending of a string two semitones up, then one semitone down, then one up, then two down. Sound When a string is bent, the sound it creates is much smoother than would be otherwise, even using other slurred techniques such as hammer-ons, pull-offs, or finger slides. String bending on the guitar was first used in blues to mimic the smooth sound of a slide guitar. It has since become an integral part of playing lead guitar. Some masters of string bending on guitar include David Gilmour, Tony Iommi, Brian May, T-Bone Walker, B. B. King, and Eric Clapton. To facilitate his extensive string bending, Clapton used to substitute an unwound banjo string for the third string on his guitar. At that time, no set of light-gauge strings with an unwound third string was available. Keyboard instruments Finger vibrato is also a standard part of clavichord technique, known as Bebung. Until the first half of the 20th century, the clavichord was the only keyboard instrument on which finger vibrato was possible. In 1928, Maurice Martenot—inspired by his experience as a cellist—invented the Ondes Martenot, which featured a keyboard that the player could rock back and forth laterally. Other finger vibrato techniques may also be used on pressure-sensitive electronic keyboards with appropriate sounds and patches. For example, some Rodgers digital church organs have an optional voice for the upper keyboard