It was claimed in another subreddit that most standing armies during the time of armored knights (15th-16th centuries I suppose although it wasn't explicitly stated) forbade commoners from attacking enemy knights and royalty, and that knights wouldn't generally attack the peasants as much has seek out other knights to fight. How much truth is there to this, was it common? If so, when the knights of one side defeated the knights of the other did they then turn to attacking the commoner forces or just sit back and watch the rest of the battle?
No for a few reasons.
A) a rule like that would be almost impossible to enforce. "Was this knight killed by that commoner with a spear or that knight with a spear."
B) if you enforce this rule than 90% of your army is just going to be cut down by a few dozen knights.
B2) if this was the case Battles would only be fought by knights. We know they weren't.
C) commoners wanted to live just like modern soldiers. It's hard enough to stop a conscript from running away. If you tell him he can't defend him self for a certain enemy he's going to run off.
D) if you enforce this rule and you enemies don't you're at a serious disadvantage.
E) because of a knights better equipment and training and the fact he always fought mounted he wasn't likely to be killed by commoners anyway.
F) chivalry was a bit of a myth. Yes knights were suppose to be chivalrous but they were people, they lied and cheated just like anyone else. And commoners couldn't give to shits waver they were considered honourable, they just wanted to get back to their family's.
G) say you do enforce this rule and all you knights and all your commoner get used to not having to worry about each other in battle. What happens when you fight the Muslims or pagans or any group that doesn't have knights but still has mounted troops? You knights are going to get killed by the foot troops and your commoner are going to be killed by the mounted troops.
After the 16th century guns became the main weapon of war and with a early firearm you didn't get to pick who in that group of 500 people you hit. You are right that knights would seek out other knights but that's because the only people who can go toe to toe with a knight is another knight or equivalent.