The picture below shows the organisation structure of a Roman Legion. The explanations are in Dutch, so I will translate them for you:

Het Romeinse Leger: the Roman Army

legionarius x 8* = contubernium: 8* soldiers form a contubernium, a eating and sleeping unit.

contubernium x 10 = centuria: 10 contubernia form one centuria, a unit of 80 soldiers**

Leider centuria = centurio, assistent = optio, veldtekendrager = signifer: a centurio is the leader of the centuria, his assistant is called optio and the bearer of the sign of the centuria is called signifer.

centuria x 6 = cohort, ervaren kampcommandant = praefectus castrorum: 6 centuriae form a cohors (or: cohort, but "cohors" is the Latin name), i.e. 480*** soldiers, an experienced leader of the whole camp = praefectus castrorum (he has been upgrades out of the ranks of the centuriones).

Cohort x 10 = Legioen, Leider = Legatus, assistent = Tribuni: 10 cohortes form a legion; the leader of the Legion is called Legatus (he is the formal "general" of the men; the praefectus castrorum is in charge of the daily organisation of life within the camp, food & beverage, clothing, housing, etc.); his assistants are called Tribunus (Militum; plural: tribuni militum).

N.B.: *8 soldiers, 10 at the most

**80 soldiers, 100 at the most

***480 soldiers, 600 at the most.

A total Legion could count up to 6000 soldiers, in some periods of the Roman Empire even more.

Tanaquil