The original SENSE-ation Harness is a no-pull dog harness that works wonders for positive reinforcement dog training, or just taking gentler walks with your pooch. An instruction booklet comes with all SENSE-ation Harnesses. The SENSE-ation is similar to the SENSE-ible Harness in design, however the SENSE-ation uses a custom Softouch polyester webbing for added comfort. The SENSE-ation Harness also features Duraflex plastic hardware, instead of metal. The O-rings on the chest and sides are nickel plated steel for extra durability. The curved side-release buckle and strap keepers are a plus. The girth strap, also called a "belly strap" comes in a coordinating color, making it easy to identify, and therefore easier to put on. The SENSE-ation Harness stops your dog from pulling, without choking, stress or otherwise harming your pet. You can use i right away - there is no acclimation period. Best of all, the SENSE-ation Harness is a favorite of positive reinforcement dog trainers and humane societies worldwide. Awarded "Gear of the Year" by The Whole Dog Journal, 2003. Using a flexible measuring tape, measure from just behind the front legs all the way around your dog. You are measuring the circumference of the body just behind the front legs, like performing a chest or belt measurement on a human. X-Large 1 Inch Width 39 - 50 Inch girth*. Large - 1 Inch Width 32 - 40 Inch girth*. Large - 3/4 Inch Width 31 - 40 Inch girth*. Medium/Large - 1 Inch Width 27 -32.5 Inch girth*. Med /Large - 3/4 Inch Width 26 - 32 Inch girth*. Medium - 1/2 Inch Width 24 - 28 Inch girth*. Small - 5/8 Inch Width 20 - 24.5 Inch girth*. X-Small - 7/16 Inch Width 16.5 - 20.5 Inch girth*. Mini - 7/16 Inch Width 13 - 16.5 Inch girth*. * If the measurement falls within two harnesses, we suggest the larger size. Example: 20" girth = Small.