Before plate armor, people had to wear chain mail.  Chain mail is more expensive and harder to make than plate armor.  Only nobility could afford plate armor, because it was still costly.  Plate armor is hard to make, just not as difficult as making the tiny rings for chain mail.  Plate armor was invented  by 1420 ad.  The main purpose is to give a knight more protection from enemy weapons.  Plate armor is more flexible than chain mail and is lighter.  Different countries had different styles for plate armor.  It was rarely used by the 1600s because of the  invention of the firearm.  The firearm was more popular than earlier technology such as swords and spears because firearms are easier to train someone to use.  Plate armor is my favorite invention I learned about this week because in my favorite video game you can craft plate armor.