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. They will brace against a source of pressure which is why dogs lean against collars. It is important to measure your dog carefully and order the proper size. Using a flexible measuring tape, 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. 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).