Long, long ago when books were bought in shops and there was no net, I read a piece that said that the Romans, being the sly foxes they were, had left-handed cohorts for their flanks.

In combat all of those right handed men drifted to the right. The cohort on the right stopped this from happening and the one on the left could more easily turn the flank of the enemy.

I don’t know that this was a common practice, it was just written.