This brings up a fourth principle related to the third: protective gear always is a compromise between protection and mobility. One could indeed be completely covered in metal but wouldn’t be able to move.
Thus, we have the openings in the armpit, the groin, the knees and the neck, simply because these places need to move to let the wearer move and fight. The only way to protect these areas is with flexible material such as mail.
Therefore, knights would take their swords with two hands and drive them forcibly in these places. It was no easy task and fights could last several minutes. A good way to take down an opponent would be to wrestle them to the ground and open the visor to stab them in the face.
Gruesome, but isn’t it better than dying because you didn’t raise your shield at the right moment, and got your sword arm cut off? The chances of survival are still way higher with full plate armor. Then why didn’t most foot soldiers in the Middle Ages wear plate armour then?
Full plate armor limits vision, hearing and proper breathing. Most soldiers who wore full plate armor thus were soldiers on horse back, and only lowered their visors before going into a charge. Common foot soldiers preferred flexible and simple armor like brigandines, which are flexible plates riveted together inside cloth. Helmets like the kettle helm, visorless bascinets and sallets were also popular because breathing and vision weren’t impaired.
Yes, you risked getting an arrow in the face, but at least you were able to see what was going on and stay in your formation.