What is the origin of the idiom "by a nose" and when to use it?

The idiom "by a nose" is believed to have originated in the world of horse racing, where it is used to describe a victory that is achieved by the smallest of margins. In a horse race, the winner is typically determined by which horse crosses the finish line first. If two or more horses are very close, the winner can be determined by a photo finish, where a camera captures the moment when the horses cross the finish line. In this case, "by a nose" refers to the fact that the winning horse's nose has crossed the finish line just before the noses of the other horses. The phrase has since been used more broadly to describe any situation where a victory is achieved by a very small margin, such as in sports, politics, or business.