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Grammatical theories of Scalar Implicatures make use of an exhaustivity operator exh, which asserts the conjunction of the prejacent with the negation of excludable alternatives. We present a new Grammatical theory of Scalar Implicatures according to which exh is replaced with pex, an operator that contributes its prejacent as asserted content, but the negation of scalar alternatives at a non-at-issue level of meaning. We show that by treating this non-at-issue level as a presupposition, this theory resolves a number of empirical challenges faced by the old formulation of exh (as well as by standard neo-Gricean theories). The empirical challenges include projection of scalar implicatures from certain embedded environments (‘some under some’ sentences, some under negative factives), their restricted distribution under negation, and the existence of common ground-mismatching and oddness-inducing implicatures. We argue that these puzzles have a uniform solution given a pex-based Grammatical theory of implicatures and some independently motivated principles concerning presupposition projection, cancellation and accommodation.