The pitch of the strings can then be continuously varied by moving the slide up and down the neck. The usual limitation in fretted guitar playing of twelve pitches per octave does not apply. Indeed, in pure slide guitar playing the frets serve no purpose, other than as a visual reference. The technique lends itself to glissandi (swoops up or down to a note); in addition it has the ability to evoke sounds of the human voice, crying, sighing or weeping, or natural noises.
Many slide guitarists will still use their free fingers to fret the strings if they want to employ that sound as well. Using the free fingers opens up the possibility of playing chord shapes other than the straight line given by the slide. One strategy is to use the free fingers for rhythm work, and intersperse this with lead phrases played with the slide.
It was invented by Brian Cober, a Canadian blues musician. In double slide, the first slide is placed on the middle finger (usually a modified steel bar that can be put on the finger) and a modified thumb slide is put on the thumb that is able to cover two strings. Double slide is meant to be played on a six string lap guitar (or a regular six string guitar modified with the strings raised up for high action like a lap guitar), usually tuned to open e tuning. The double slide guitar system allows for the player to achieve chords not heard in open tunings, such as minor chords, dominant seventh chords etc. and allows for a greater use of technique in soloing. Jonathan St. Rose "Brian Cober, blues guitarist" Will Ray of the Hellecasters uses a similar technique, wearing "stealth" pinky type slides on either hand.
The first musician to be recorded using the style was Sylvester Weaver who recorded two solo pieces "Guitar Blues" and "Guitar Rag" in 1923. Some of the blues artists who most prominently used the slide include gospel singer Blind Willie Johnson, Blind Willie McTell, Mississippi Fred McDowell, Son House, and Robert Johnson as well as Casey Bill Weldon of the Memphis Jug Band. The sound has since become commonplace in country and Hawaiian music. It is also used in rock, by bands such as The Allman Brothers Band, Lynyrd Skynyrd, Led Zeppelin, Pink Floyd (whose original singer Syd Barrett used a Zippo rather than an actual slide on the song See Emily Play), and ZZ Top that have been heavily influenced by the blues. While the Allmans were the first to bring slide guitar to a wide audience, The Rolling Stones used it as early as their 1963 recording of the John Lennon/Paul McCartney song "I Wanna Be Your Man". Guitarist Brian Jones played slide in a very blues-oriented style. His successor, Mick Taylor also displayed his own slide guitar skills while with the band, using a bottleneck on studio recordings and during live performances.
Blues legend Muddy Waters was also very influential, particularly in developing the electric Chicago blues slide guitar from the acoustic Mississippi Delta slide guitar. Texas bluesmaster Johnny Winter developed his distinctive style through years of touring with Muddy. Slide player Roy Rogers honed his prodigious skills by touring with blues giant John Lee Hooker. John Lee's cousin, the great Earl Hooker, may have been the first to use wah and slide together.
Second is Elmore James's famous cover of the riff from Robert Johnson's "Dust My Broom", a textbook example of slide guitar in electric blues. Finally, a part of Duane Allman's solo from Eric Clapton's "Layla" is included, to give an impression of highly acclaimed slide work in rock music.
Source: Wikipedia > Slide Guitar
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