Don’t forget VimL, the language that insanely decided to use double quotes as a “start of comment” indicator, then went back and gave them special meaning based on their position so they either signal the start of a comment or a string.

A different comment character could have been chosen, but this affects nothing. It's not insane.

Lua, and there are others. I favor double quotes for this same reason even when single quotes are allowed.

I'd like «guillemets» but that's going to have to be self-serve...

I use single quotes when the string in question is used like an enumeration:

    set_color('black','white')

And double quotes for strings meant to be read by a human:

    print("Hello there.  How are you?")

And an example with both forms:

    syslog('warning',"unit %s spin rate %f too low",u,rate)

For me, it's an indication of how I expect the string to be used.

And even then, Perl and Ruby (heck, even Bash) have a semantic difference between double- and single-quoted strings. The only other popular general-purpose languages where they're synonymous are PHP, JavaScript, and Lua.

>The only other popular general-purpose languages where they're synonymous are PHP, JavaScript, and Lua.

PHP does treat single and double quotes differently. Contents of single quotes strings are not parsed for variable substitution.

Also Pascal and SQL.

And JavaScript!