Bloodhounds are distinguished from other breeds by their extraordinary olfactory powers, which enable them to hunt entirely by air-borne scent. They are the oldest breed of sporting dogs which hunt by scent, and Bloodhound Trials enable the hounds to demonstrate the remarkable abilities for which they have been bred - a fine illustration of dogs. Bloodhound Working Trials are open to purebred Bloodhounds only. The trials are a challenging and exciting countryside activity in which hounds follow a human scent trail over a 'line' up to three miles long. The line has been laid by a 'runner' up to two hours previously, following a pre-determined route set by the judge and marked on a map. When 'hunting the clean boot', as it is termed, the hound must follow the scent, ignoring all distractions, and at the end of the line it must go up to and positively identify the runner, who will be standing in a line-up with two other participants. Identification usually takes the form of two large (muddy) paws placed on the runner's chest!