I know that the original three lines were not compatible with each other. They are the VPI line, the Snakekeeper line and the Jolliff line. There is a NERD line that I believe is compatible with the VPI line and there is the Marcus Jayne line (that I mentioned before) but I think it's pretty new so I don't know if anyone has tried to cross it with another line yet or not. There are probably others but those are the ones I know about.