Before the development of the telescope, the known, visible planets were assigned rulership of signs. Each planet was considered to rule two signs (the Sun and Moon are usually only assigned one each, though recent scholarship suggests that they may have in ancient times been given co-rulership of each other's signs, Leo and Cancer). Some modern astrologers assign rulership of certain signs to the recently-discovered planets, Uranus, Neptune and Pluto, but others allow them co-rulership with the traditional rulers. Traditional astrologers however deny them any rulership, even if allowing them some influence.

This definition is in context to Astrology. See more contextual defintions for Co-ruler.