What is Trigraph characters is C?

Trigraph characters is C

• The C character set is contained within seven-bit ASCII.
• In order to write programs in the reduced set, corresponding single characters replace the occurrences of the trigraph characters.
• The replacement occurs before any processing.

Trigraph charager replaced by
??= #
??/ \
??’ ^
??( [
??) ]
??! |
??< {
??> }
??- ~