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The Elliptical is a 45 mm lozenge, it is stronger than the Control Plate due to it having a smaller surface area. Consequently, a horse using his tongue as a means of pushing against the bit and therefore taking control, will draw back and tuck his chin in, coming back to a better point of control.
The Petros mouthpieces rotate preventing the horse taking hold of the bit. Often good for a horse that leans or tries to take hold of the bit.