Tapping is a playing technique (generally associated with electric guitar playing, though the technique can be performed on any string instrument) executed by using the fingers of the picking hand to tap the strings against the fingerboard, sounding notes. Tapping (also known as a two-hand hammer-on, or the redundant "fingertapping"), performed in conjunction with normal fingering by the fret hand, facilitates the construction of note intervals that would otherwise be impossible using the fretting hand alone. Tapping usually incorporates pull-offs as well, where the finger that just tapped the fingerboard to sound a note is then swept off with enough lateral motion to sound that same string again � this time on a lower note than the tap (fretted by another finger before the pull-off, or simply left open).
Perhaps the most famous employment of tapping is the short piece "Eruption" on the first Van Halen album, which features very fast tapping triads and formed the blueprint of heavy metal lead playing throughout the 1980s. While Eddie Van Halen is generally credited with inventing tapping, the practice had existed in some form or another for centuries; Paganini utilized similar techniques on violin. A similar technique is used in Turkish folk music on ba�lama, which is named selpe. Credit for the first application of this classical technique to popular music may more accurately be given to Steve Hackett of Genesis, who used the technique both live and on recordings in the early 1970s. Ace Frehley of KISS has used tapping on solos before Van Halen's debut, as has Angus Young, although he usually does one-handed tapping (such as at the end of the solo to "Dirty Deeds Done Dirt Cheap").
A related technique is "tapped harmonics", where the fret hand acts as a barre, while the harmonic is tapped. Eddie van Halen does this in the acoustic guitar solo "Spanish Fly".
Stanley Jordan uses a style similar to tapping called "touch guitar", where the pick hand is used to play lead while rhythm is played simultaneously with the fret hand.
The Chapman Stick is an instrument built exclusively for tapping. The Warr guitar is also built for tapping, though not exclusively. Both instruments use lower string tension and low action to increase the string's sensitivity to lighter tapping.
An earlier recorded example before Van Halen is on Orchestra Luna's only album Orchestra Luna, on the outro to "Doris Dreams". Randy Roos is the guitar player.
Even in the '80s hair metal in which tapping had its heyday, tapping on the bass guitar was rarely heard, the most famous practitioners being Billy Sheehan and Stu Hamm. Michael Manring brought the technique to New Age music in his early recordings with Windham Hill. Dream Theater bassist John Myung frequently uses this technique on bass guitar.
Two-handed tapping is rather a progressive approach to play polyphonic/homophonic music on a guitar by using eight (and even nine) fingers. Two-handed tapping applies the concept of playing a piano. For example, the right hand plays the treble melody while the left hand plays accompaniment. Therefore, it is possible to play piano scores (like J.S. Bach's two-part invention). The advantage is that it enhances the flexibility of playing a guitar, while the main disadvantage is the lack of change of timbre. Eddie Van Halen incorporated this technique into modern rock guitar with the solo from "Judgement Day", and patented a pop-out stand to use to employ this technique. Adam Fulara inspired a playing method called "Portato" to two-handed tapping. It can increase the ability to control the change of dynamic for tapping. Since the first note usually sounds the loudest, (unwanted in some music like jazz), controlling of the dynamic is an important topic in two-handed tapping. T.J. Helmerich is another two-handed, 8-finger tapping guitarist. You can hear his work on albums by Garsed&Helmerich and on the Uncle Moe's Space Ranch albums.
A outstanding master employing exclusively this technique to an original blend of mainstream jazz, Turkish, Uzbek and Balkan folk music and classical music is Enver Izmailov.
During the 1980s two-handed tapping had developed much further with players such as Nuno Bettencourt and Steve Vai using multiple strings and fingers to stretch over multiple octaves.
Modern bands like The Dillinger Escape Plan often use extreme form of tapping. This form can be found in many of their songs, good example of which can be heard at the beginning of their debut album Calculating Infinity where the guitar players use fingertap with 3 fingers on each hand. (Courtesy of Wikipedia)
