A trigraph is a three-character replacement for a special character. A trigraph can be used in place of a symbol that is not present on a keyboard or in a character set.
(Note: Some programs cannot properly interpret trigraph)
Trigraph Character
?? --> #
??( --> [
??) --> ]
??< --> {
??> --> }
??! --> |
??/ --> \
??' --> ^
??- --> ~
However trigraph is not just limited to the computer. In a language a trigraph is a set of three consecutive letters that represents a single sound. For example, in the term "bureau of investigation" the letters e, a, and u together represent a long "o" sound, as in "oh."