Traditionally, English saddles have two flaps on either side of the seat with the girth and the stirrup leathers sandwiched between the two. A mono-flap saddle only has one flap, and they have longer billet straps, so the girth buckles don’t sit under the rider's leg.  With a mono-flap saddle, there’s less bulk under the saddle flap so it gives you more contact with your horse, which makes them a good choice for dressage riders. Mono-flap saddles are easier to clean and give you a deeper, more secure seat, but they're usually more expensive.