This is a simple A Type Harness - your dog simply steps in, you fasten the clip, attach your lead and you're ready to go. They are much more comfortable to your dog than just a collar pulling on its neck. The Harness is made from a 25mm polypropylene webbing for extra strength and durability, polypropylene is rot proof, U.V resistant and doesn't retain water. They are also resistant to most common chemicals. The size is for the girth of the dog (around it's chest and back): Medium - 18-23" (46-59cm), Large - 20-27" (51-69cm).