Please check the image in the product web page and product description part, which will teach you how to train dog to accept muzzle. Because dogs are anxious about wearing muzzles at the beginning: try to take off muzzles or bite through muzzles NOTICE: Place your forefinger under the tape measure to ensure enough space for your dog to be comfortable. Your dog must be ableto pant and have free tongue movement. Notice: Not suitable for pit bull mix and all other dogs with a shorter snout. MEASURMENT: Measure circumference of dog' mouth and dog neck. Please check the second picture and choose suitable size .Please double check before purchase. HIGH QUALITY: Made of durable, high-quality nylon that can stand up to daily use and is comfortable to wear. Large Size - Snout strap 7-11.8 in, Neck strap 6.3-14 in. Suitable for normal size Dobermann, Collie, Pit Bull and so on.