A martingale collar has a second, smaller loop that tightens when your dog pulls back, just enough to stop them slipping out, then relaxes again. They are the usual choice for greyhounds, whippets and other dogs whose heads are narrower than their necks, and for any dog who has learned to back out of a flat collar.

Fit it so that at full tension the two rings do not touch; it should sit loosely the rest of the time. Clip the leash to the D-ring on the small loop, and take a martingale off when your dog is crated or unsupervised.