The single Dreamscreens come with the deep top guide rail, which was initially developed for the double doors to add strength and stability.  The deep top rail prevents wind from blowing the screen out of the rail in anything but severe conditions.  Because the top and bottom rail are not the same depth, the standard Dreamscreen now must be specified as either left-hand or right-hand (see image below).

The graphics aren't pretty, but we're engineers, not artists!!

The old standby for determining the correct hand is the "butt to butt" test:  Open the existing door, stand in the doorway with your back against the hinges (face the door latch).  Which ever hand swings towards/with the existing door is the type of Dreamscreen you need. 

Deep top rail

The exclusive deep top rail design provides increased strength and greatly improves resistance to wind blow out.