Horses are herd animals, and in their natural environment seem to be able to run and wheel about in a mob without striking each other's heels. However, in a horse race, when a human is in charge of the steering mechanism, a horse's natural ability to avoid heels appears to be affected by the introduction of this human interference with nature.

In fair racing each horse must be entitled to gallop without restriction in the ground in which it has established itself.

But some riders are inclined to disregard caution when endeavouring to move in closer to the rails in order to save ground on turns, or to move in or out to take advantageous positions at critical stages of a race. It causes danger to the lives of horse and rider when the rider of one horse takes it across the running of another. There is the likelihood of the outstretched front legs of the rear horse clipping the extended hind legs of the horse in front. Even the possibility of the front legs of a horse being contacted is a recipe for disaster and loss of life or serious injury. The potential sudden fall of a horse in these circumstances makes reckless, close crossing one of the most dangerous actions in horse racing.

Therefore, since racing first started there has been a constant debate on what is a safe and practical clearance in these circumstances. The standard adopted by the Racing Stewards in Hong Kong is at least two lengths clear that is, the length of the horse crossed plus a length.

The clearance space required in front of the horse being crossed is the distance to be taken up by its outstretched front legs, plus the distance taken up by the extended back legs of the horse crossing, plus a safety clearance margin of at least a foot. For practical purposes, this distance is about the length of a horse standing still. The crossing rider also has to ensure that his shift takes into account the possibility of his mount's front legs contacting the heels of the horse racing ahead of the space he is intending to fill.

A length in racing is recognized as the length of a horse standing still, and that "two-lengths-clear" is taken by racing people and the public to mean "a horse's own length plus another length clear of the horse whose running is being crossed".

The requirement could be expressed as "one clear length ahead of a horse before the commencement of any move across its running".

A horse needs at least one clear length to accommodate its front legs and the outstretched heels of the horse in front. In many places around the world this is not considered satisfactory, however, with the competitive style of racing in Hong Kong this is a minimal distance for safety.