The term is particularly common in older breeds for which a written breed registry was not created until after the breed phenotype was well established. However, many modern breeds can be traced to specific, named foundation animals.
The terms for foundation parents differ by sex, most commonly foundation sire for the father, and foundation dam for the mother. Depending upon the species in question, more specialized terms may be used, such as foundation mare for female horses, or foundation queen for female cats. Less common is foundation bitch for female dogs.
The first generation progeny of foundation stock of a hybrid are referred to as F1 hybrid animals, the second generation as F2, and so forth, usually through F4.
In dog breeding, a foundation sire is a male progenitor of a bloodline, while the corresponding term for the female is foundation dam (or, in a declining usage, foundation bitch).
The American Kennel Club operates a breed registry for over 60 nascent and experimental breeds, called the Foundation Stock Service Program (FSS), through which breeders can seek to establish full AKC recognition of their new breeds.
In horse breeding, the terms foundation sire and foundation mare or dam refer to the earliest progenitors of a breed. There are usually few foundation sires, but several mares, though in many cases, foundation mares are not always identified in old pedigree records, though in Thoroughbred breeding, pedigrees are traced to the tail-female line. In most cases, breeds that require that all members trace to specific foundation stock have a closed stud book and do not allow crossbreeding to other animals. The Thoroughbred, Andalusian and Arabian are examples of breeds with a closed stud book.
Some breeds, such as the Morgan horse, have a single named foundation sire, while others, such as the Lipizzan, may have several. In some cases, particularly with older horse breeds, some or all foundation sires may be unknown. However, in breeds with a well-documented breed registry, all or nearly all foundation animals may be identified. For example, there are three major foundation sires of the Thoroughbred, and another 24 or 25 minor foundation sires, along with 74 foundation mares. An example of a foundation bloodstock pedigree line within a breed are the Crabbet lines from the Crabbet Arabian Stud farm in England. These animals were bred by the same program for 92 years, were exported worldwide, and had a substantial impact on the breed. Some Arabian breeders today specialize in horses descended only from this breeding program. Similarly, in the Standardbred, the Clay Trotting Horses constitute a distinct foundation line within that breed.