David Gilmour performing an extreme bend on several strings.
A bend is a technique where a fretting hand finger is used literally to bend the string up or down (toward either side of the neck). The bend creates an increase of tension on the string and therefore an increase in pitch. Although bends are possible on a variety of instruments they are most commonly associated with the guitar, especially as played in rock and blues.
The top three strings are the ones most often used for bending. The leverage between the third finger and the thumb gives the pressure needed, especially if the thumb is hooked over the top of the neck. The string should be held down with the middle of the fingertip so that it doesn't slide out underneath or catch on the fingernail.
On most guitars, the second and third strings are the easiest to bend. Although it can be bent in either direction, pushing a string upward (toward the bass strings) generally allows better control than pulling it down (toward the treble strings). When a string is pushed up, the whole hand can get behind it; when its pulled down, one finger has to do all the work.
Bends can vary in extent from a half-tone (semitone) to four half-tones. There is also a bend commonly used in blues guitar playing, called a blues curl, which is a bend of roughly a quarter-tone (in other words, a microtone).
Hitting the target pitch accurately is vital when bending. How far a string has to be bent to hit the right pitch depends on the string gauge and the position on the neck. Getting a feel for the relationship between pressure and pitch comes through practicing and listening.
Avoiding unwanted noise
Accidentally hitting other string with the finger that is doing a bend can result in unwanted noise, ruining an otherwise good solo. There are several ways to avoid this noise. One is to deaden other nearby strings by muting them with the first or second finger, behind the fret on which the string is being bent. Another is to push the nearest strings clear with the protruding tip of the finger that is doing the bending.
Bends using other fingers
Having mastered bends of up to three half-tones with the third finger, it is time to learn how to use the others. As with all guitar techniques, being able to use all the fingers equally well increases versatility.
The second finger should not prove too difficult once the hand has been adjusted so that the thumb still provides a fulcrum for the necessary leverage. Bends with the first finger can present problems: first, the fulcrum of the thumb is not as effective; second, there is finger free to deaden strings behind the bend; third, the first finger is usually needed to play other notes either immediately before or after the bend. However, it should be possible to use the first finger for bends of a least a half-tone on the second and third strings. Fourth finger bends are obviously more difficult, since the fourth finger is much weaker than the others.
All string bends can be done equally effectively in reverse. In other words, starting with the string bent up to a higher note, it is struck and then relaxed back to its normal pitch. The effect is of a single note falling in pitch. As when bending notes up, the speed at which the pitch drops can be varied. Descending string bends, or pre-bends, sound particularly good if vibrato is applied to the note being bent down to.
Pre-bending demands a good feel for tension and pitch, since the note can't be heard before it's played. The solution is to practice, first with a one-fret bend, then with two, and then with three, to learn the differences in feel. A good approach is to play a single note and bend it up and down two or three half-tones while it is sounding. Bear in mind that the tension differs from string to string.
It is possible to bend two strings at once to produce a slightly discordant effect which can nevertheless sound good in the right context. The most common double-string bends are on the second and third strings, bending both strings from the same fret, and on the first and second strings, bending the second string from one fret higher than the first. However, the technique is usually more effective if one string is bent and the other isn't.