A barre chord, or bar chord, is a specific type of fingering designed to change the key of a guitar chord without changing the chord pattern. It is made by placing the second, third, and fourth fingers of the left hand in the desired chord pattern while placing the index finger firmly across the fret directly below the fingering. The index finger changes the length of the strings, which affects the pitch of the notes. Using this technique, the same chord shape can be used to play a greater variety of chords.
The effect of a barre chord is often compared to a capo. A capo is a device which is affixed to the neck of the guitar, changing the length, and therefore pitch, of the string. The root note of the string becomes the fret where the capo is secured. For instance, if a capo is affixed to the neck at the third fret, the root is G on the lowest string. This allows the guitarist to transpose the key of a song without changing the chord pattern.
Acting like a capo, the index finger presses all or some of the strings down to change the root note. It requires a great deal of finger strength to hold the strings down properly. If the pressure is uneven or inconsistent, the sound will be out of pitch and distorted or might not even sound at all. It takes practice to master a barre chord.
Guitarists use barre chords because they can play multiple keys without changing their pattern. For instance, the most commonly used chord patterns are E major and A major, and placing the E major chord three frets higher and barring on the third fret will create a G major chord. This is a short cut, saving the time it takes to learn the open string, or non-barre, pattern for the chord. The barre chord, though versatile, is not a panacea. It cannot be used to play every single chord, but is simply one method of creating a chord.
Popular music, beginning with the blues in the 1910s and 1920s, and continuing into the influential music of the 1950s, used simple chord progressions of three or four chords. Barre chords are often used in this type of music. Not only do they make key changes easy, but they often allow beginning musicians to play entire songs using only one fingering pattern.
Rock music of later decades began to use the "power chord". Also a type of barre chord, the power chord is fretted off with the index finger, but without a full chord pattern. Only the root and fifth of the chord is fingered. It is typically played with an amplifier or special effects pedal to create distortion, making the guitar sound fuzzy or gravelly, like a television that isn't picking up the correct channel.