The Protocols of the Great Houses were the laws they followed.
However, since the Great Houses essentially created history, many of the Protocols were hardwired into the very structure of the universe itself. Sometimes they could be bent, as in when pre-War and post-War groups met. An example was Chatelaine Thessalia encountering a future Faction Paradox expeditionary force on Zo la Domini. (PROSE: The Book of the War)