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Auxiliary units


Auxiliary units

Military ranks


Dates and events


Weights and measures

Auxiliary units

Auxiliary soldiers were organised in infantry units, cohorts (singular cohors, plural cohortes, abbreviated to coh) and cavalry units, alae (singular ala). Infantry units could also sometimes include a cavalry element (cohors equitata), sometimes described as ‘part-mounted’ regiments. There were two sizes of both cohors and ala, quingenary (quingenaria) and milliary (milliaria), the former with a nominal strength of 500 men, the latter with a nominal strength of 1,000 men, although in practice these figures are different.

The main infantry subdivision of the cohort was the centuria containing 80 men, itself divided into 10 sections (contubernia). The term centuria was often symbolised by an abbbreviation resembling a 7. The main subdivision of the ala or of the cavalry contingent in a cohors equitata was the turma probably consisting of 32 cavalrymen.

The table summarises the theoretical composition of these different units.

Summary table

Unit Subunit No. of subunits Number of men per subunit Total no. of men
Ala milliaria turma 24 32 768
Ala turma 16 32 512
Cohors milliaria equitata centuria & turma 18 (10 centuriae, 8 turmae) 80 and 32 800+256
Cohors milliaria centuria 10 80 800
Cohors quingenaria equitata centuria & turma 9 (5 centuriae, 4 turmae) 80 and 32 480+128
Cohors quingenaria centuria 5 80 480
Legion cohors + centuria 10 cohortes, each of six centuriae (1st cohort of double strength) 80 c.5000

The numbers, size and type of barrack blocks are a guide to the garrisons that occupied forts, along with other evidence such as inscriptions. At Wallsend for example the eight barrack blocks correspond well to the requirements of a cohors quingenaria equitata, of 480 men and 120 cavalry, six barracks housing the infantry and two the cavalry. In many cases however the numbers and size of barrack blocks do not fit the standard pattern, suggesting that parts of several units may have been brigaded together. Individual units were perhaps in any case only rarely at their full theoretical strength. 154, the strength report of the first cohort of Tungrians, a cohors milliaria peditata illustrates well the potential differences between the 'paper' strength of a unit and its actual strength.

For further information see The fort plan and Vindolanda units and their origins in the exhibition.

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