Breeds that require that all members trace to specific foundation stock have a closed stud book and prohibit or strongly limit crossbreeding to other animals.
These animals were bred by the same program for 92 years, were exported worldwide, and had a substantial impact on the breed.
Similarly, in the Standardbred, the Clay Trotting Horses constitute a distinct foundation line within that breed.
In other cases, where a breed or landrace is older than any written records, the foundation bloodstock is sometimes described by myths or legends, such as the mythical horses of Mohammad, known as Al Khamsa ("The Five"), claimed in legend as the foundation mares of the Arabian horse breed.
The word may refer to animals tracing only to a select subset of the oldest foundation bloodlines, particularly when newer breeding has been added that changed the original phenotype.