I don't know what they are called, but i found some good pdf describing them some time ago, now it's lost. I have been searching for "modular systems" but I haven't found anything of worth. Is that their real name or are they called anything else?
Edit: Oops, I obviously forgot the most important part...
Edit: The next thing to do when I have find a good-looking symetrical solution for this systemis to solve this system: