Late Roman Unit Sizes

[Robert Vermaat]

Most of these terms were used interchangeably - Vexillatio becomes both a generic term and a specific regiment in the 4th century. Legion and Cohort were both used generically but were specific regiments.

Vexillatio was not a generic word - it referred specifically to a cavalry unit of the field army. While certain writers (particularly very late ones like Claudian and Gildas) used 'legion' and 'cohort' to describe units generically, we're talking here about official military nomenclature, not literature.

Numerus was used, it seems officially, to refer to many different types of unit, from legiones to auxilia to equites and scholae. So it could not have referred to a single fixed number of men, unless we assume that all these units were the same size.

It is possible that the numeri of auxilia were originally formed with the same organisation as a cavalry vexillatio or schola - they seem to use what might be a new cavalry rank structure, for some reason - but even so this would not explain why legions were also called numeri in the ND and the Perge inscription.

Vexillatio became a word for a cavalry unit ion the field army (don't know exectly when)  but before that it was a generic word for any number of troops dispatched from a parent unit.
Numerus may have become a more generic word for different unit types, but also originated as a word for an unspecified number of troops.

The thing is, the Roman sources that we have don't use the same words for the same subject, which leaves us guessing. This goes for military equipment as much as it goes for military organisation. Frustrating as it is, the harder we try to make sense of this the more we run into a dead-end street I think. trying to give hard definitions to loosely used terminology is doomed to fail.

I agree with Nathan that there is no reason to look for a 'hidden' or 'secondary' army organisation behind the one we have. A numeri (not numerii) organosation for understrength army units would be extremely complex for modern armies, it would be totally confusing for anyone in the Roman army.

Why would temporary units need a fixed number of troops? Did the US Army fix the numbers in their task forces? Did the German Wehrmacht limit set number limits in their Kampfgruppen? Roman vexillations and numeri were every bit as much created for special needs during special times, no matter that sometimes such units 'solidified' into standing units, and the names stuck. I don't see any reason to look for a special army organisation where it's not needed.