Why backquotes in GNU documentation?

[Dave]

[Not sure which newsgroup would be best, sorry for the spam.]

Hi,

I have often seen single backquotes in GNU software documentation. For
example, in http://www.gnu.org/software/libc/manual/html_mono/libc.html
one can read:

        Using `htonl' is necessary so that

or:

        followed by lower-case `a'

Error messages printed by GNU software also often contain single
backquotes. For example, in commands.c of GNU make 3.82:

            error (NILF, _("*** [%s] Archive member `%s' may be bogus; not
deleted"),
                   on_behalf_of, file->name);

Source code comments also have single backquotes. For example, in
commands.c of GNU make 3.82:

  /* If the target is an archive member `lib(member)',
     then $@ is `lib' and $% is `member'.  */

And even in the title of http://www.gnu.org/software/make/manual/make.html:

        GNU `make'

Now, I was wondering what could be the reason behind this convention.
Why not use just straight quotes also in front of the quoted word? For
example:

        Using 'htonl' is necessary so that

or:

        Using "htonl" is necessary so that

-- dave