Hello, I am Charlemagne and I am the emperor and I used to be friends with the pope but now we have different views. He thinks that I am supposed to help him with doing God's work, but I know that he is supposed to help me with this! After all, I am the emperor!
I am the pope and I issue a set of rules: Only I can appoint bishops, and only I can dethrone emperors.
hrmm, like i'm going to follow his rules!
I appoint you bishop of Milan!
Can't only the pope do that?
I excommunicate you from the church!
I am going to try to remove you as pope!
I forgive you, King Henry!
Later, though, they forgave each other.
Only Popes can appoint Bishops, but Emperors can give them fiefs!
Even later, in 1122, the Concordat of Worms was signed between Kings and Popes, but they still did not get along after this.