Get the current addons directory?

How would you go about getting the path for the addons directory used by the currently running Blender instance? It seems if I use ([:-11]) I could get the root directory and then just generate the rest of the path from there, but I only know where it would be on Windows and I’d rather a method that reliably gives the complete addons directory for all supported operating systems (Windows/Linux/OSX).

A list of addon paths