Chest piece. Suggested sizing for the following breeds as a guideline only: XL: Newfoundland, Bernese Mountain Dog, Mastiff Large: GSD, Greyhound, Rough Collies, Large Border Collies Medium: Springer Spaniel, small to medium Border Collies, Cocker Spaniel, Whippet Small: smaller Terriers, Dachshund, CKC Spaniel, King Charles Spaniel, Toy / Miniature Poodle