 Like other second-order linguistic conventions, conversational implicature conventions differ in their cross-linguistic spread. Whereas the quantity implicature of “some” has been found in all observed languages (Horn 1989), common metaphors differ as much from language to language as the idioms that evolve from them do. The fact that regularities in implicature obtain cross-linguistically is compatible with their being arbitrary and conventional. Witness the practice of using “?” and rising intonation to mark questions. Why some implicatures are common to more languages than others is an open question on any view. To the extent that implicatures are conventional, it is a question for historical linguistics.