origin of the backtick for quote

Kevin Buchs, asks in gnu.emacs.help about the origin of the convention of using backtick and straight quote, `like this', in GNU documentations, in particular, emacs. Quote:

Ok, dumb question to which I have been unable to find the answer and which is distracting me:

In emacs documentation, what is the origin of using the accent grave (backtick) to introduce a quoted phrase, often a command, while using an apostrophe to terminate it.  Example: (info) Keys and Commands: 1st paragraph: "binding" is quoted as such, but 2nd paragraph, `next-line' is quoted that way. If someone who knows the answer will take the time to answer, I promise I will document it on the Emacs wiki. Does this extend beyond emacs? Beyond GNU & FSF?