legion marching formations

[D B Campbell]

When the enemy was near and could endanger the marching army a formation called agmen quadratum was used. Although the term is not very clear it seems quite probable that this was in fact a formation of three parallel columns marching side by side, from which the army could very quickly form the acies triplex.

Agmen quadratum just means "squared up", so it suggests two parallel columns with a space in between.