Side reins are a training aid used when lunging the horse. Side reins imitate the contact that a rider would have on the bit by attaching one end of the reins to the girth and the other end to the bit on either side of the horse.  The elasticated section of the side rein allows for elasticity and softness, encouraging the horse to release and soften onto the bit. Side reins aren’t used for riding, though some riders do use this when schooling a horse.