Why do Unix man pages use double backticks in place of double quotes?

I've noticed that man pages and other documents formatted by Unix utilities often use double backticks followed by double single quotes '' to wrap quoted phrases instead of the double quote character ". Single quotes are similarly replaced. Why is this? Here are a couple examples, from the man page for grep: To find all occurrences of the pattern `.Pp' at the beginning of a line: $ grep '^\.Pp' myfile The apostrophes ensure the entire expression is evaluated by grep instead of by the user's shell. The caret `^' matches the null string at the beginning of a line, and the \' escapes the .', which would otherwise match any character. The grep utility is compliant with the IEEE Std 1003.1-2008 (``POSIX.1'') specification.