The term “Great House”, or “Sector House”, refers to those civilian organizations that have a presence across the Calixis Sector. This sets them apart from “planetary” or “lesser” houses, which might wield enormous power on a particular planet but do not have any particular influence elsewhere. In the earliest days of the Calixis Sector, all the Great Houses, and most of the planetary houses, were based around noble families. The old Great Houses were enormous extended familial lines, with strict rules of heredity to maintain their monopoly on power.
In the current days, several of the Great Houses are actually corporations. They may act as if they were noble families but the truly hereditary Great Houses are quick to point out that their memberships are based on ambition and skill, rather than bloodline. The Great Houses play their own game of politics and wealth, and with their thousands of members and troops, they have the potential to exercise enormous power in the Calixis Sector. Some dominate whole worlds and some harbor ambitions that will one day pit them against the Adepta of the Imperium.