The Market Harborough, named after the town of it's origin in England, is a form of martingale, but is unsafe to jump in. It has a very precise action that only works when the rider chooses it to, and so relies on rider skill. When the horse has his nose perpendicular to the ground, the Market Harborough should be clipped to the appropriate rein ring, so the horse will be comfortable when he has his nose in that position. It will not be possible for the horse to raise his head but it doesn't make him overbend. It is very useful in skilled hands for as short a period of time as possible.