we advise you to base your choice mainly on the dog's weight. The half shoulder blade size is specified for information only as the harness does not follow the dog's shoulder along its entire length.
Measure the half shoulder blade size, from the sternum (pointed bone in the middle of the chest) to the withers (junction between the neck and the top of the shoulders), on one side only.
If your dog is between 2 sizes, choose the smaller one.
An X-Back harness should fit snugly on the neck and extend to the base of the tail with a medium pull on the cord at the back of the harness (it's normal for the harness to be slightly loose on the dog when not pulling).