They're a familiar sight on chat boards, in spam messages and in viruses. Even a co-worker might use one - jokingly, of course. They're words that look unpronounceable: "l33t," "w00t" and "h4x0r," among many others.

They're all part of "l33tspeak" (pronounced "leet speak"), Internet slang that at one time identified the writer as a proficient hacker and now identifies anyone who uses it seriously as a hopeless wannabe.

L33tspeak started in the 1980s in the hacker community. Some say it was born of a need to avoid the prying eyes of keyword searches, while others say it was really just a form of graffiti-like expression in a drab, text-based world.

"It's a very quick way of identifying who's in your gang," says Graham Cluley, senior technology consultant for security vendor Sophos.

In the late 1990s, use of l33tspeak made its way into online chat boards and online games. Since then, it's been overused - mainly by teenagers trying to win respect among hackers - to the point where it has become a source of amusement or annoyance.

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L33tspeak leaves a lot open to the writer's creativity, but there are certain rules of thumb for cracking the code.

The first and most basic rule of l33tspeak is to change certain letters to similar-looking numbers or symbols - for example, "e" becomes "3," "a" becomes "4" or "@," and so forth.

The second rule is that certain letters need to be transformed into something cooler, 'Nettier - and frankly, more l33t. If a word ends in an "s," it probably ought to end in "z" instead, so "wares" becomes "warez" - or, more correctly, "w4r3z." The letter "x" is eminently more l33t than "ck"; with another tweak or two, that's how "hacker" becomes "h4x0r."

Again, there's a lot of flexibility available here, and the writer could choose to stick with letters and symbols where the substitutions are fairly easy to recognize - or the writer could go with an "advanced" l33tspeak, where the characters are "drawn" rather than substituted.

For example, "M" could become "|\/|" and "U" could become "|_|" (see chart for some examples of character representations).

It seems pretty straightforward - until you see /-\ |_0+ 0|= +3X+ ("a lot of text").

"It does require skill to read and write quickly, especially when you get away from close representations of actual letters," says Elias Levy, architect of DeepSite services at Symantec.