Was there a second in command in medieval battles?

Answer 1

Sometimes, but usually not. Because battles were mostly conducted by Royalty, there would usually only be someone to take the place of a killed or injured leader if there were other Royals on the field. There was nothing like the chain of command and the organised leadership transfers that there are in today's armies

To expound on the above, Western medieval combat was basically a semi-organized mob. Each side would consist of several groups, each usually commanded by the noble from whose lands the soldiers came. There would be a single "commander" of the whole army, but this person was often not in operational control of the army. There did not exist any sort of chain-of-command; rather, the assemblage of nobles would usually pick a leader (and often a deputy) to be in overall command. This commander could then issue what we would now call directives (rather than orders) to each of the nobles' groups.

For instance, here's how a medieval army might be "structured":

King A brings 100 knights, 1000 man-at-arms, and 500 archers.

King B bring 200 knights, 5000 man-at-arms, but no archers.

Noble C (who nominally owes fealty to King B) bring 100 knights.

Noble D (an independent noble who isn't royalty, but isn't a vassal of King A or B) bring 500 man-at-arms and 500 archers.

A, B, C, and D would all retain "command" of their individual groups of soldiers (it would be very unlikely to do something like combine the archers of A & D into one group under someone else's command). Probably, King B sould be selected as the leader of the whole army. King A would likely be elected deputy commanders. In all likelihood, though, Noble C and D would NOT take "orders" from A unless they knew that B had been killed or otherwise out-of-action.

If that sounds horrible, it was. Medieval battles were notoriously uncoordinated and chaotic, with bad battlefield communication technology making the situation even worse.

Formal chain-of-command structures didn't start occuring until the end of the medieval period, when warfare shifted towards nation vs nation rather than traditional feudal warfare. That is, things didn't really improve until the Renaissance.