What is the difference between a King and a California King?

Sounds like a riddle, right? Well, it's much simpler than that. 

A King (or Eastern King) is the widest mattress size. It measures 76 inches wide by 80 inches long. 

A California King, on the other hand, is the longest mattress size. It measures 72 inches wide by 84 inches long. Note that is it longer, but more narrow that a standard King.