Interpreting Polybius' description on a formation

[Bryan]

Polybius in fact refers to two different three feet spaces: 1) the space occupied by the soldiers themselves and their equipment and 2) the intervals between soldiers. While this has often been read as producing a file width of 6 feet, these two spaces likely overlap. That is, the interval between soldiers of three feet is likely calculated from the center of the soldier, who occupies part of it with his person and weapons. The actually file-width would only be 4.5 feet (or so--no need to be dogmatic about what foot Polybius is using, although its probably Attic).

This might explain how the Romans actually "found" their space when the created formations. The Roman shield is somewhat under 2.5 feet in width (once you factor the curve), and its may be that the Romans created their "open order" formations simply by starting off with locked shields, and then having every other man step forward, which would create shield sized intervals between the men of the front rank.

I discuss this more here (available in full on line): https://www.academia.edu/3394799/Visual_...ry_Tactics and here (PM me for a full PDF) https://www.academia.edu/1602947/Roman_I...assessment

You've emphasized the importance of the simplicity of drill movements in terms of forming a battleline from marching column in your previous writing, whether coming from camp or after a period of travel. To expand on that, I'm trying to wrap my head around how a Roman century or maniple comes to form line from column. 

If someone is marching in formation, front to back, each man needs about an arms length reach of space between them or else they step on each other's heels. Since there is no proof of Romans calling any cadence to march in lockstep, it means the distance between each man in column would need to greater than arms length to prevent even the possibility of the trailing left foot from the man in front connecting with the advancing left foot of the man in rear, causing a "flat tire" incident, resulting in a very pissed off individual missing some skin on the back of his foot. And the faster they walk, the more they stretch out their step, the farther they need to be. So a minimum of arm's reach between them. 

I just grabbed up my wife for an experiment. I had in her front of me, we both faced to the forward, me at her and her at the wall, we did the "cover" portion of the D&C drill maneuver to Dress and Cover. I pushed her forward as I extended my arm until she no longer touched my arm. That simulated us being in a marching column mimicking it already having hit the pivot point at the far left of the designated battlefield, performing a column right, marching until the position the tribune's pick for them at which point the century/maniple is halted (roughly a maniple gap in between), being faced left toward the enemy, and being ready for battle. Doing that we ended up in line, and by spacing we were roughly an arm's length away. Roughly 2.5 feet. Or the width of a scutum. 

So take a century in marching column and halt them, have them face left, and they're in Polybius' open ranks. 

To go from that position to close interval/to remove the gaps between members of a rank, the second rank can do what you state, to take a step forward to fill in the gap, which results in a thin formation of only 2-3 ranks but one that maintains the same frontage. The other possibility is they keep the same frontage width by going from open interval to close similar to how the US military drill and ceremony does it, everyone on one wing of the rank (the left in US mil) moves toward the other side to fill in gaps, by using elbows or some other means of gauging how close they are from one another. While this would result in the frontage of the formation shrinking, it maintains the depth of the formation.