Mixing and matching these conditions produces sixteen possible combinations, of which eleven are valid Creative Commons licenses and five are not. Of the five invalid combinations, four include both the ""nd"" and ""sa"" clauses, which are mutually exclusive; and one includes none of the clauses. Of the eleven valid combinations, the five that lack the ""by"" clause have been retired because 98% of licensors requested Attribution, though they do remain available for reference on the website. This leaves six regularly used licenses: