ROT13 is the fixed 13-position Caesar substitution for the 26-letter Latin alphabet. It transforms only A-Z, preserves upper- and lowercase, and leaves digits, punctuation, whitespace, and other scripts unchanged.
Each letter index becomes (index + 13) modulo 26. Because two shifts total 26, the same operation encodes and decodes; it is useful for hiding spoilers or puzzle text, but offers no cryptographic security.
Hello, World! -> Uryyb, Jbeyq!
Uryyb, Jbeyq! -> Hello, World!