The pastern strap is designed to work as a safety strap if the boot comes loose or gets stuck in mud etc. The strap is not the main component keeping the boot in place. Made from durable (TPU), the strap is flexible and gentle against the horse's skin when fastened correctly. The strap is adjustable. Four pastern straps (two pairs). Size guide: Small Fits size M00-00. Medium Fits size 0-3. Large Fits size 4-9. Extra Large Fits Scoot Boot size 10-11. To secure the strap correctly, make sure the horse is standing straight and bend its leg 90 degrees. Tighten the strap and make sure there is room to fit one large finger or two smaller fingers at the front of the buckle.