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(Good god, I can't believe no one has noded this yet..)

Tablature (tab for short) is a form of musical notation which works best for fretted string instruments like the guitar or bass guitar, although there are adaptations for other instruments. Most of the following is going to deal with guitar tablature, since that's what I'm familiar with, and it's not to hard to see how it can be applied to similar instruments.

Tablature for the guitar consists of six lines, one for each string of the guitar. The standard tuning for guitar strings, from lowest tone to highest (thickest string to thinnest) is E A D G B E. A bit of tablature might look like this:

(For those interested, that's the F# major scale)

The string each line represents is denoted by the letter at the beginning, with a lower-case 'e' denoting the high E string (usually the thinnest string on the guitar), and an upper-case 'E' denoting the low E string (usually the thickest string on the guitar). The tablature is read from left to right, with each number representing the fret to be played. In the above example, the guitarist would first play the second fret on the low E string, then the fourth, then the first fret on the A string, and so on. Numbers which are directly above or below one another are supposed to be played simultaneously. Sometimes the space left between notes is supposed to denote timing, but this is difficult to do when dealing with ASCII text, so it's not too common and usually not exact.

Various symbols are used to denote certain 'effects' and picking styles. They tend to vary from tab to tab, but the most common ones are:

x - pick scrape; also sometimes used to show muted strings in a chord.
m - palm mute
^ - bend the string
. - staccato
~ - vibrato, or sometimes it just means to hold the note
/ - Slide up from the previous note to the next
\ - Slide down from the previous note to the next
h - hammer-on, as in |--4h6--|.
p - pull-off, as in |--6h4--|.

(More to be added)

Update 2001/02/05:
Lactic.Acid pointed out the use of x for the muting of a string in chords, and reminded me about hammer-ons and pull-offs. Thanks!