This prevents collar pressure on the neck of the dog if it pulls while on a leash . A harness is suitable for small dogs, while a head collar is much more effective for controlling large or, strong dogs. A head collar is made of strong nylon and is attached to the leash under the dogs chin. If the dog pull or lunges forward on the leash, itís forward movement pulls its head down and itís Jaw shuts. This is ideal for a large dog that may be difficult for its owner to keep under control in public.