The French Spaniel (Epagneul Français) was developed in France as a hunting dog, descended from dogs of the 14th century. (Source: AKC)
"If not the oldest, the French Spaniel is certainly one of the oldest, pointing dog blood lines. This handsome spaniel first gained fame during the Middle Ages as a setting dog, utilized both for net hunting and falconry. As direct descendants of the famous "chien d'oysel" these bird dogs - described by Gaston Phébus are the progenitors of virtually all of today's hunting spaniels."
National kennel clubs and breed clubs (see, e.g. Breeding/Health Strategy Documents, below)
Population-level statistics (see, e.g., Swedish Insurance Data, below)
Breed club surveys
There are numerous breed standards.
The basis of breed/conformation shows is the judging of pedigree dogs against the 'Breed Standard', which is a picture in words that describes the range of features that are deemed appropriate for the breed.