Thundershirt"s patent- pending design applies gentle,constant pressure on a cat"s torso, and this pressure has an amazing calming effect for most cats. Pressure has been used to successfully reduce anxiety for many years for both animals and humans. Examples include parents swaddling infants, veterinarians using squeeze chutes when vaccinating cattle, and thousands of people with autism using pressure to relieve persistent anxiety. Use Thundershirt For: Vet Visits Fearfulness Car Travel Reactivity Grooming Litter Box Problems Noise Anxiety Crating. SMALL - CHEST SIZE 9" UP TO 13" - WEIGHT MEDIUM - CHEST SIZE 13" UP TO 17" - WEIGHT 9 - 13 LBS LARGE - CHEST SIZE 17" UP TO 24" - WEIGHT > 13 LBS