Made from tough twin ply nylon with chromed steel joins and a quick-release catch. The special locking system gives additional stability over traditional harnesses. These harnesses have alot of adjustment making them suitable for growing dogs. The are also particularly well suited to broad stocky dogs. How to measure up:harness sizes are based on your dog"s "girth" measurement, see diagram below. The harnesses are adjustable, so select the size nearest your dog"s measurement.