The Great Houses began as various departments within the Imperial Bureaucracy. During the Isolation, they slowly seized control, absorbing other departments, consolidating power; eventually cementing themselves into positions of great authority within the city.
Each of the Great Houses controls a District of Rastaban, renamed in their honour.
Membership of a Great House can be hereditary, if the Head of House approves it.
Whilst many citizens may belong to a Great House in name, far fewer are part of the group that effectively holds power within that House.