dee it looks like they just added the back side on each each end. say the distance between the legs (outside of each leg is where you measure) is 16" and the leg where the overlap is an inch. You would cut that 18" plus your seam allowances. I hope that makes sense.

Thanks, Lizzie. I'm not sure I completely understand, though, but I'm going to reread it several times until I do. :)

I think I see what you mean by the slip cover is wrapping around the corners. I think the draping part of the slip cover on the sides are longer than the seat cushion part so go around the back legs of the chair so the back "drape" on the cushion over laps it. You could make the side drapes longer than the cushion part and the back drape go over it and attach together right at the top back of the leg with a small piece of of velcro for easy removal or a few hand stitches. If I could draw a picture it would be easier.

Thanks for the help Izzi; I got a few books from the library so I hope to come up with a picture there.
Dee

To me it looks like they added a gusset on each corner. That would be a 3 or 4 inch piece for each corner where the legs are and then sewed the sides to meet in the center of the gusset. Much like a pleat on a sofa or chair skirt