What's the difference between a flat collar and a martingale collar?

A flat collar buckles or clips to a fixed circumference, while a martingale (no-slip) collar tightens a limited amount when a dog pulls back, making it harder for narrow-headed or escape-prone dogs to back out. Reviewers in r/dogs and r/reactivedogs frequently recommend martingales for that reason, though many prefer versions with a quick-release buckle for easy on/off.