With our unique fit and leash methods, your dog feels the harness against her sides (not against her legs). This helps him use her sense of touch to sense how to follow your leash. It is similar to how a horse responds to a rider's leg pressure since both horses and dogs have a similar response to sense of touch. Measuring: It is important to measure your dog carefully and order the proper size. To get the correct size, use a flexible measuring tape and measure from just behind the front legs all the way around your dog, similar to a belt measurement on a human. This is your dog's girth measurement. Please choose the size that best fits your pets girth. Mini: 7/16 Inch Webbing (13-16.5" girth) X-Small: 7/16 Inch Webbing (16.5-20.5" girth) Small: 5/8 Inch Webbing (20-24.5" girth) Medium: 3/4 Inch Webbing (24-28" girth) Med/Large: 3/4 Inch Webbing (26-32" girth) Medium/Large: 1 Inch Webbing (27-32.5" girth) Large: 1 Inch Webbing (32-40" girth) X-Large: 1 Inch Webbing (39-50" girth).