But it was not only the patrician class which Augustus endeavoured to preserve; the ancient class distinctions among the citizens were respected as far as possible. The senators, raised in public esteem by the expulsion of unworthy members, wore even under the principate the broad purple hem as a mark of their rank; they had special seats reserved for them in the theatres, and received from Augustus the privilege that the crimes of senators could only be judged by the senate itself. They could contract legal marriages with none but freeborn persons. In like manner the knightly class was purged of unworthy elements and maintained as a distinct order with a fixed income and recognised privileges. As in republican times, the younger members served as a _guarda nobile_, being mounted on chargers provided by the state in the field and in the gorgeous processions on civic festivals. The knights were eligible for all curule offices and military appointments, so that the order became the nursery for the military and civil service as well as for the senate. Augustus chose his provincial procurators and tax-collectors by preference from among them. The emperor endeavoured to preserve even the free burgesses from the admixture of alien elements as far as possible, and to this end imposed restrictions and limitations on the manumission of slaves. [Illustration: ROMAN DOOR-KNOCKER] As commander-in-chief of all the military forces, and head of the senate, Augustus was master and ruler of the state; but one important element of the power which Cæsar had wielded was still lacking--the tribunician authority. This also was conferred upon him for life by the senate and people in the year 23, in the general rejoicings at his recovery from an illness, and because he had appointed L. Sestius, the friend and comrade of M. Brutus, to a share in the consulate.