The drum set in early stage did not have a hihat cymbal. It is said that Baby Dodds - well know for New Orleans jazz in 1920s - is the first player who used it. At that time, 2 cymbals were set just above a pedal so that the player can step on the pedal by the left leg and make a sound. It was called "Low Boy", "Sock Cymbal" at that time. Later on, it was set higher in order to hit it with a stick. It was like "a hat in the high position", so people started to call it "Hihat". I think hihat is one of the few instruments that can make the sound groovy by moving the instrument itself.

This is how hihat cymbals look like.

Glance from the side.