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As opposed to fans who are particularly attached to one brother or the other, or to only [[Sam]] and [[Dean]], or just [[Castiel]], tribros are devoted to the relationship between all three.
It arose after the eleventh season finale of ''Supernatural'' when Dean clearly stated that Castiel is a brother, by fans who wanted to be clear they had affection for all three brothers.
Even though Dean has said Castiel was like a brother to him before, in [[6.20 The Man Who Would Be King]], Dean said in [[11.23 Alpha and Omega]]: "You're our brother, Cas. I want you to know that."