At 1.9 cubic inches, it is by far the smallest you can buy
Attaches to your dog's existing collar - (one inch width or less.) No need for your dog to wear a second collar
Adjustable sensitivity levels so you can be sure to detect your dogs bark every time
Uses loud but harmless audible sound to deter barking
Lifetime Warranty

"In a Tuskegee University study of adult shelter dogs wearing the PetSafe Bark Control Collar for six 30 minute sessions over two weeks, we detected no long-term adverse affects, and all dogs dramatically reduced their barking by the second day."
Dr. Janet Steiss and Dr. Caroline B. Schaffer
Tuskegee University