[split] Distances between files and ranks

[Nathan Ross]

an army commander would only have an army that was extremely small in size and the space it occupied.

Not that small, surely. A legion drawn up eight men deep, with every man occupying a square metre and no gaps between units, would have a frontage of 600-700 metres (depending on legion size). Not inconsiderable on the pre-modern battlefield, and most armies would comprise more than one legion, plus auxiliaries etc.

In any case, we know from Arrian that 8 men deep was possible - all we are discussing is whether it was usual. Or, indeed, whether there was a usual...

Are you saying that the entire legion, or entire army, would be drawn up 2 men deep in a long line - or that the army/legion formed perhaps a triple acies, with each line (acies) two men deep? That would give you six men deep in all, plus gaps between the lines, and you'd probably end up occupying almost the same amount of depth, or more, than a single 8-deep formation and no greater amount of frontage overall.

What period are you referring to about a century training together? In 462 BC...

Certainly not that early! All my considerations, unless stated otherwise, relate to the army of the Principiate, with men mustered into standing units and remaining in their centuries for perhaps their entire military career. The situation in the militia-style army of the early Republic would be quite different, of course. The majority of our sources on army organisation seem to fall between the two periods!

when in march column, with the infantry four wide and the cavalry in a single line on either flank of the infantry, the allocated number of cavalry matches the unit’s depth

I do not think that could possibly work. Horses advancing in a line need to have 4 feet between them (half a horse's length; each horse is about 8 feet long, so each occupies c.12 feet of road) - any closer than that and they will kick and shy. You cannot very easily force horses to walk nose to tail.

A marching man, on the other hand, needs only a single pace distance between him and the men ahead and behind. So unless you want to open 12ft gaps between your marching men, to match the spacings of the horses to either side, the cavalry cannot possibly 'match the unit's depth'.

Four deep infantry for me, and four legions.

This would be a legion in single column marching four abreast, then, with three more legions moving up to form lines on the right flank, each marching four abreast, and then wheeling to form a battle line four deep in four lines?

So the whole army would be drawn up in four lines, each line a single legion arrayed four men deep - sixteen men deep in total, with spacings between the lines?

Would that be about right?

In fact what Metellus is doing I have found to be much simpler and discussed in some battle accounts.

I don't know - it sounds pretty simple already to me!

He provides the number of men in the fifteenth legion simply by mentioning the five centurions of the first cohort.

Arrian does not say that there are five centurions in the first cohort. He says nothing about the number of centurions.