Slide Guitar, Double 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.

Source: Wikipedia > Slide Guitar