Chirp Back Wheels System. This bundle comes with four Chirp Wheels in sizes 4", 6", 10", and 12", a carrying case for all of them, and the posture corrector. Used small wheel only twice. Posture corrector was only tried on, never used. Excellent condition. Pet friendly/smoke free home Purchasing this item acknowledges that you have viewed all photos, read the full description and have asked all the necessary questions prior to buying.