As regards the rate and method of promotion, and the order of precedence of the various posts within the two groups of _principales_ and _immunes_, we know practically nothing. There is nothing to show that it was customary to hold several posts in a regular order,[120] or to become an _immunis_ before entering the _principales_. It was doubtless usual for a man not to receive commissioned rank without first holding some subordinate post, but we do not know that any such preliminary qualification was essential.[121] Owing to the length of service promotion was probably not rapid, but on the other hand the number of posts available was very large. In an _ala quingenaria_, for example, there were 16 decurions, 34 _principales_, and probably over 100 _immunes_.[122] Thus every soldier must have felt confident of obtaining sooner or later a position of greater ease and profit, and this, together with the fact that the ladder of promotion led to commissioned rank, and even to the coveted legionary centurionate, must have increased the attractions of the profession.
_Titles of the regiments._ The titles of the auxiliary regiments were as various in form as those of the legions, and it is unnecessary to give a complete list of them. The alae which bear a title derived from a personal name, presumably that of their original commander, have been mentioned already. The majority of them were probably raised during Caesar’s Gallic campaigns or the Civil Wars, and there are few to which a later date can be assigned with any certainty.[123] The few cohorts known to have borne such titles are more difficult to explain, but have perhaps a similar origin.[124] Regiments raised under the Empire, on the other hand, were usually called by the name of the tribe or district from which they were raised, and distinguished by a number from other corps of the same origin.[125] In course of time these ethnical titles were in many cases supplemented by others, some of which were granted as marks of distinction and rewards for meritorious service, while others were purely descriptive. Examples of the former class are the title _civium Romanorum_, which indicates that on some occasion all the members of a corps received the franchise before their discharge,[126] and honorary epithets, such as _pia_, _fidelis_, or _fida_.[127] The title _Augusta_ seems also to have been granted at all periods _honoris causa_, although some of the regiments bearing it may date back to the beginning of the Empire.[128] Titles derived from the names of later emperors, on the other hand, while they were doubtless granted occasionally as marks of distinction, seem often to indicate nothing more than the reign during which a regiment was raised. Finally, from the time of Severus Antoninus onwards, every regiment employs a secondary title, derived from the name of the reigning emperor. A remarkable series of dedicatory inscriptions of the Cohors I Aelia Dacorum, which was stationed during the third century at Birdoswald (Amboglanna), on the British frontier, shows us this regiment successively assuming the titles _Antoniniana_, _Gordiana_, _Postumiana,_ and _Tetriciana_.[129]