pattern - How did I split the alphabet?

I have split the alphabet in two sets, as you may see in the picture below, according to a very specific rule. Which was the rule I used?

Answer

The top set of letters are:

Letters that have no curves but do have "branches" (ie cannot be drawn without lifting a pen off the paper or retracing parts of the letter)

The bottom set are those left over.

This does I think generate the correct sets (I'll be very embarrassed if I am wrong on that) but it feels a bit too complicated to be an elegant answer...