Doggles ILS are the first and only protective eyewear designed just for dogs! 16 years of experience has made our Doggles ILS (Interchangeable Lens System) the very best in pet eyewear. For true eye protection or fashion, your pet deserves the very best. Doggles offer 100% UV protection, shatter-proof lenses, and anti-fog treatment on the inside of the lenses. Lenses on the ILS style are interchangeable and replaceable. This style is suited to all dog head shapes and sizes. Sizing: To determine proper size, measure the entire head circumference across the eyes around the entire head under the ears. To determine proper chin strap length, start measurement just under one ear, go under the chin and up to bottom of other ear. Extra Small - For dogs 1 to 10-pound / Back strap and frame length - 5 to 13-inch / Chin strap - 3 to 7-inch. Extra Small Breed example: Chihuahua, Pom. Small - For dogs 9 to 25-pound / Back strap and frame length - 12 to 20-inch / Chin strap - 4 to 8-inch. Small Breed example: Beagle, Westie. Medium - For dogs 20 to 60-pound / Back strap and frame length - 15 to 25-inch / Chin strap - 5 to 11-inch. Medium Breed example: Dalmation, Border Collie. Large - For dogs 50 to 100-pound / Back strap and frame length - 18 to 30-inch / Chin strap - 7 to 16-inch. Large Breed example: German Shepherd, Labrador Retriever. Extra Large - For dogs 100 plus-pound / Back strap and frame length - 22 to 45-inch / Chin strap - 10 TO 24-inch. Extra Large Breed example: Large Rottweiler, St. Bernard *Longest strap measurement is length fully streched. Please keep in mind that breeds vary by size and that the breeds above are examples, you should go by the size and weight of your dog. The weight and frame measurement are the most important measurements.