The Plaid Step-in Harness from Daba Doo is the best-fitting step-in we've found. The front and back straps separate right at the clasp at the dog's shoulder blades - nothing impeded the dog's movement by going down the shoulder. Easy to use and fit, the Plaid Step-in Harness comes in sizes to fit almost any dog - from 1 1/2 lb. Ava (next to the box of tissue), to 8 lb. Jack the Yorkshire Terrier, to 19 lb. Booker the Boston Terrier.
We call it the Plaid Step-in because many years ago, when we first started carrying this harness, all the the colors/patterns available were plaid. The line has expanded, but we haven't changed the name so the people whose dogs' harnesses finally wear out after a decade or more can still find them!