The beautiful Isle of Bute formed part of the domain of Walter, the first High Steward, and remained a Stewart possession except for a brief Norse occupation. But only after 1385 did a family branch become established there, when Sir John Stewart a son of King Robert II was appointed hereditary Sheriff of Bute and Arran; and his descendants still hold the marquisate of Bute.
Septs of Clan Stuart of Bute
Tartan of Clan Stuart of Bute
The spelling Stuart originated with some Stewarts living in France where the alphabet has no `w'. Adopted there also by Mary Queen of Scots, it became fashionable when she continued using it on her return. Steuart was a compromise between the two forms.
[From "Scots Kith & Kin"]
Through history, Clan Bannatyne was a sept of Clan Campbell; also a sept of Clan Stuart of Bute.