Two-handed tapping, sometimes known as Octodigital Tapping, is the act of fretting notes on a guitar with both the fretting and picking hands. It's an extension of the hammer-on and pull-off techniques commonly used with the fretting hand, which can provide counterpoint notes at a range impossible to reach with one hand. The speed at which rolls can be performed with two hands is much greater than what can be achieved with one.
Classical and jazz musicians such as Nicolo Paganini, Chet Atkins and Barney Kessel occasionally used techniques similar to the two-handed tapping method seen in modern guitar styles. Bassists have been practicing Emmett Chapman's parallel "Free Hands" technique since the late 1960s. Richie Blackmore, Duane Allman, Frank Zappa and Brian May all dabbled in the techniques. It wasn't really until the 1980s, and the advent of "shredding," that two handed tapping became common on the guitar. Eddie Van Halen, Steve Vai and George Lynch all contributed to the popularity of the technique during this period.
Two-handed tapping can be performed on any standard guitar, but several instruments have been produced specifically to allow advanced two-handed techniques. The Chapman Stick and Mobius Megatar are both bass/guitar hybrids with upwards of eight strings, designed to be played with both hands on the fretboard. Vigier prodcues a more traditional guitar with a totally flat fingerboard as a signature model for tapping virtuoso Stanley Jordan. According to UltimateGuitar.com, players like Greg Howe often mute the open strings by wrapping a damper or tissue around the guitar's zero fret.
There are a variety of ways to approach tapping, besides the traditional "Free hands" method, where the fingers are parallel to the frets. For quick flurries of tapped notes within a traditionally picked piece, players will often tap with the edge of a plectrum. This makes switching between styles more seamless and the tapped notes sound more aggressive. Steve Vai can be heard doing this in "Eugene's Trick Bag" from the movie "Crossroads."
Van Halen's "Eruption" is one of the first tracks containing prominent two-handed tapping to gain mainstream popularity, and the technique gained further exposure with Eddie's solo in Michael Jackson's "Beat It." "The Final Countdown" by Europe contains an example of Octodigital tapping in the middle eight, where guitarist John Norum plays rapid notes using all eight of his available fingers.
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