"Duces" differed from "praesides" who were the supreme civil as well as military authority within their provinces in that the function of the former was purely military. However, the military authority of a "dux" was not necessarily confined to a single province and they do not seem to have been subject to the authority of the governor of the province in which they happened to be operating. It was not until the end of the 3rd century that the term "dux" emerged as a regular military rank held by a senior officer of "limitanei" – i.e. frontier troops as opposed those attached to an Imperial field-army ("comitatenses") – with a defined geographic area of responsibility.