The holders of subordinate posts, who ranked below the centurion or decurion, may be divided, following the arrangement adopted by von Domaszewski, into two groups.[82] The members of the first group practically correspond to our non-commissioned officers, and are able to command small detachments or to take the place, if necessary, of the company officers. These alone, and the holders of certain higher administrative posts, to which the _taktische Chargen_ gave access,[83] have a legitimate claim to the title of _principales_. The members of the second group did not, strictly speaking, rank above the privates, but they were granted freedom from certain routine duties in return for special services which they discharged, and were distinguished in consequence by the title of _immunes_. It is of course often difficult to ascertain whether a particular post falls into the higher or lower group, and this is especially the case with the standard-bearers, who occupy a position of peculiar importance in the military system. In the ala each troop had its own flag carried by the _signifer turmae_, but there seems also to have been a regimental standard, the bearer of which was known as the _vexillarius alae_.[84] A few inscriptions also mention an _imaginifer_, but it is not clear whether this officer always or at all periods found a place on the staff.[85] In a cohort, on the other hand, each century seems to have had its _signifer_, and each turma of mounted men its _vexillarius_, but it does not appear that there was a regimental standard, any more than there existed at this date a standard for each cohort of a legion. This at least is implied by Tacitus in his description of the entry of the Vitellian army into Rome, when he mentions the _alarum signa_ by the side of the _legionum aquilae_, but says nothing of the ensigns of the cohorts.[86] We must suppose, then, that the _imaginifer cohortis_, who is mentioned on inscriptions, was not regarded as the regimental standard-bearer any more than the _imaginifer legionis_.[87] In consequence of this difference in organization the company and troop standard-bearers of the cohorts rank among the _principales_, while in the alae only the regimental standard-bearer is included in the higher group, and the _signiferi turmae_ sink to the position of _immunes_. Returning, then, to the ala we may place at the head of the _principales_ the _vexillarius_, and next to him the _imaginifer_, when this officer existed. Other members of this class were the non-commissioned officers of every turma, the _duplicarius_ and _sesquiplicarius_,[88] who derived their titles from the fact that they were paid twice and one and a half times the private’s pay respectively, an institution found in the Hellenistic military system from which it was probably borrowed.[89] Lastly we should perhaps add the _optio_, who commanded the escort of the _praefectus_ (_singulares_).[90]