Hell of a Summer
Simply because a person does not go by d’Cannith or d’Deneith does not mean they are not recognized members of that House. Instead, it simply means that person does not bear the Dragonmark of their House. Each Dragonmarked House is made up of a few major and several minor families but only recognized members of a house with a Dragonmark are entitled to use the prefix “d’”.
For example, Lady Elaydren Vown d’Cannith, bearing the Mark of Making, may go by Elaydren Vown, Elaydren d’Cannith, or Lady d’Cannith. Similarly, un-Marked Lord Aron Fostern is still a recognized member of House Deneith even though he is not entitled to calling himself Aron d’Deneith.
Members of the traditional nobility and royal bloodlines use the prefix “ir’” to denote their status. The most famous of these is the royal family of Galifar: the ir’Wynarns. Members of the Dragonmarked Houses are prohibited by the Korth Edicts from marrying into the nobility or gaining noble titles in any of the former kingdoms or protectorates of Galifar, so no member of a Dragonmarked House is legally entitled to that designation.