Why do Kings go by just their first name, whereas modern heads of state tend to go by their last name?

by iswater

For example, we have King George or Queen Elizabeth. On the other hand we have President Obama. What historical development contributed to this phenomenon?

Searocksandtrees

Hi, the reason is that monarchs often didn't/don't have surnames. There's room for more contributions on this topic, but for now, there are a few responses in this old thread