Acies quadrata

[Flavivs Aetivs]

http://classics.mit.edu/Tacitus/histories.html

EDIT: @Mark Hygate

The common consensus is that Vegetius was a bureaucrat who was writing a piece designed to make the "old days" look better. The only troops he saw were probably protectores (a unit which was nothing more than a placeholder for people earmarked for future office, like Aetius), and the local Vigilies (Town Watch). The former never saw combat and never wore armor, the latter were peasants, not real soldiers.