High boy vs. low boy

what the heck is a high boy furnace and how is it different than a low boy?

A high-boy is a tall furance with the blower under the heat exchanger.

A low-boy is shorter and has the blower in back of the heat exchanger.

Low boy-heat exchanger and blower side by side
High boy- heat exchanger and blower stacked

I wouldn’t call it “side-by-side”. Because of the burner configuration the
air from the blower has to come from behind the exchanger (or under it). Otherwise,the blower would be looking at the side of the heat exchanger. We just call them low-boys, or “basement models” around here because they were originally designed for use in a low ceiling basement.