It makes more sense if you've ever drawn in CAD. Top view, x and y. Now side view, y and z or y and x. You look down on x and y, and if you are extruding you now create the z axis dimensions. For the people who draft on the side axis: you are true psychos (ok, unless you're using a lathe I suppose, or if the silhouette is more defined from the side... ok maybe not psycho, just odd)