 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 . A becomes N , B becomes O , and so on up to M , which becomes Z , then the sequence continues at the beginning of the alphabet : 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 : 
