The Tercio was a massive formation with as many as 1,000 pike men in a large solid central block with smaller blocks of shot deployed at each corner:
So what prevented enemy cavalry from sweeping away the exposed corners of shot with a cavalry charge that avoided the central pike bloc?
Did the pikemen expand their formation in to a hollow square to protect the shot?
Did he shot retreat into the square or around the square under the deployed pikes?
And were the rear corners of shot positioned to provide all around fire power or are these the reload positions with the firing positions at the front corners?
In general, if musketeers took refuge kneeling under a protective canopy of pikes to avoid being trampled by cavalry, how may rows of shot could be protected by a line of leveled pikes?
This is a great question that I had missed for a few months, sorry!
The answer is that different elements of a tercio -- which itself is an organizational term, not a battle formation -- will move relative to each other depending on the threat.
Here is a great illustration of how elements of a pike&shot formation adjust to a threat from its right. In this case, they maximized the frontage of missile troops to pour fire onto an incoming enemy.
And to quote Lopez, Ignacio (2012-07-24). The Spanish Tercios 1536-1704 (Men-at-Arms) (Kindle Locations 772-774). Osprey Publishing. Kindle Edition.
At the approach of the enemy, the detachments of shot fell back and sought refuge within the squadron, and this was the moment for the pikemen to go into action. The posture they adopted depended upon whether they were facing cavalry or other pikemen.
Further to quote F. G. de Leon, The Road to Rocroi: Class, Culture and Command in the Spanish Army of Flanders, Brill (2009), it was the Sergeant Major who was central to "almost all of the army's decisions," as he was responsible to train the soldiers in various formations and maneuvers, and in battle he was the one to receive orders from the Maestre de Campo or Colonel, and send them to company Sergeants who lead their companies in battle.
So basically, the "perfect rectangles and squares" that you see in many illustrations were not completely fixed in battle.
