What are trigraphs chatacters?

Trigraphs are sequences of three characters in programming languages that represent a single character not easily typed on a keyboard. They typically begin with two question marks (??) followed by a third character, and are used to ensure code portability across different systems. For example, in C and C++, the trigraph ??= represents the equals sign (=). While trigraphs can be useful, they are rarely needed in modern programming due to the availability of standard keyboard characters.