Applying ROT13 to a piece of text merely requires examining its alphabetic characters and replacing each one by the letter 13 places further along in the [[alphabet]], wrapping back to the beginning if necessary.{{cite book |last=Schneier |first=Bruce |authorlink= Bruce Schneier |editor= |others= |title=Applied Cryptography |edition=Second|year=1996|publisher=John Wiley & Sons |isbn= 0-471-11709-9|pages=11 }} A becomes N, B becomes O, and so on up to M, which becomes Z, then the sequence reverses: N becomes A, O becomes B, and so on to Z, which becomes M. Only those letters which occur in the [[English alphabet]] are affected; numbers, symbols, whitespace, and all other characters are left unchanged. Because there are 26 letters in the English alphabet and 26 = 2 × 13, the ROT13 function is its own [[inverse function|inverse]]: :$\mbox\left\{ROT\right\}_\left\{13\right\}\left(\mbox\left\{ROT\right\}_\left\{13\right\}\left(x\right)\right)=\mbox\left\{ROT\right\}_\left\{26\right\}\left(x\right)=x$ for any text ''x''. In other words, two successive applications of ROT13 restore the original text (in [[mathematics]], this is sometimes called an ''[[involution]]''; in cryptography, a ''[[reciprocal cipher]]''). The transformation can be done using a [[lookup table]], such as the following: {| style="margin-left:auto; margin-right:auto" class="wikitable" | ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz |- | NOPQRSTUVWXYZABCDEFGHIJKLMnopqrstuvwxyzabcdefghijklm |} For example, in the following joke, the punchline has been obscured by ROT13: How can you tell an extrovert from an introvert at [[National Security Agency|NSA]]? Va gur ryringbef, gur rkgebireg ybbxf ng gur BGURE thl'f fubrf. Transforming the entire text via ROT13 form, the answer to the joke is revealed: Ubj pna lbh gryy na rkgebireg sebz na vagebireg ng AFN? In the elevators, the extrovert looks at the OTHER guy's shoes. A second application of ROT13 would restore the original.