Board Thread:Lore Discussion/@comment-1600847-20161103125247/@comment-26893431-20161103141116

Bronkiin wrote:

They are a pretender to the council of Great Houses. Hence rival Great House. It is revealed in dialogue and an in-game book that they have been stripped of that status. You will not find an encyclopaedia of TES that lists them as a great house.

They are a Great House. Saying they are a rival Great House doesn't disprove of this. They are unceremonically stripped of their title, but since it didn't happen in the proper way, they still are a Great House.

It doesn't matter what an encyclopedia says, what matters is what TES says.