Levinson (1995) attributes the counterfactual meanings derived from marked, periphrastic alternatives of the modal verb, could, to the presence of M-inferences related to the Gricean maxim of Manner. He accounts for the complementary nature of inferences associated with the two alternates, could and had the ability to (+ V) by the relative markedness of either expression. The present paper re-examines such claims on the basis that periphrastic modal alternates represent renewals in grammaticalisation cycles, and it is suggested instead that the Gricean second maxim of Quantity may be basic to both forms, could and had the ability to, as well as to another alternate, was/were able to (+V). A diachronic survey reveals that variation in the types of implicatures derived from such forms is due to the frequency of specific grammatical environments in which the forms are evolving, or have evolved historically.