The unique feature of this door is that it creates the illusion of an arch fireplace door in a rectangular, masonry fireplace opening. 

Converting a rectangle fireplace opening to an arched opening is not an easy endeavor, unless you install an Arch Conversion fireplace door. The main frame of an Arch Conversion door is rectangle in shape, and fits outside (Overlap Fit) or inside (Inside Fit) a rectangle fireplace opening. Then the glass and frame of the hinged door are in an arch shape, making the fire visible through an arched opening.