The marked syntactic similarities of English Tags and Parentheticals suggest that Tags themselves are a type of parenthetical. Based on this assumption a solution is proposed in which the properties of Tag form are accounted for in terms of certain non-transformational constraints required independently for parentheticals.
Studies in Language. International Journal sponsored by the Foundation “Foundations of Language” 4, 3, 379 - 409 (1980); http://dx.doi.org/10.1075/sl.4.3.04kno