Late Roman Unit Sizes

[Nathan Ross]

Now think of all the number possibilities you can get.

Why would any army have such a complicated organisational structure? And what makes you think seniores and iuniores were originally combined in the same units?

...from the arithmos of the lanciarii and the mattiari... I know the size of an arithmoi... selected from each arithmoi... no information as to the number of arithmoi of the lanciarii and mattiarii.

ARITHMOS is singular. ARITHMOI is plural. They are the same as NUMERUS and NUMERI in Latin. The numerus of the Lanciarii and the numerus of the Mattiarii are both arithmoi, or numeri.

Are these translations of Ammianus correct? Does it actually say “numerus”?

Yes.

20.4.2: Aerulos et Batavos cumque Petulantibus Celtas et lectos ex numeris aliis trecentenos

31.11.2: trecentenis militibus per singulos numeros lectis Sebastianus properare dispositus est.

The division seems quite simple to me. A numerus probably comprised c.600-800 men. Maybe up to 1200 for some legions. A detachment of 300 men is marched off to a war in the east. This detachment later forms a new unit with the additional title Iuniores, now based in the east, and is recruited back up to establishment size. The original remaining unit of 300-500 in the west is also recruited back up to size, and given the title Seniores to distinguish it from the new one. Operational strength, of course, may have remained low for some time in both cases.

At any point subunits of several hundred men can be taken from the numeri and either used as temporary detachments (as with the Adrianople example here), or combined with other detachments to form temporary mixed units for special duties (as with the Lanciarii and Mattiarii in Persia).

Very simple, very practical - a flexible military system to meet the needs of a complex era!