What is the function of a dialog? Traditionally its function is to reach an agreement on a range of knowledge to be conveyed, in order to make this shared. The language, that is a grammar shared among the interlocutors, is just the main means by which this aim is pursued. The paper investigates to what extent the metrics of speech can be induced by the pragmatic conditions of communication. The qualitative and statistic analysis demonstrates that rhythmical patterns vary according to the conversational goals of the speakers. Results are compatible with a different answer to the question with which we began. The dialog function seems mainly to realize an activity together, and such final objectives are achieved through the instrument of a mutual understanding: in other words, the dialog seems above all a cooperation, which serves the language, but which does not aim for the language.