American Quarter Horse
Since the American Quarter Horse was formally established as a breed, the AQHA stud book has remained open to additional Thoroughbred blood via a performance standard. An "Appendix" American Quarter Horse is a first generation cross between a registered Thoroughbred and an American Quarter Horse or a cross between a "numbered" American Quarter Horse and an "appendix" American Quarter Horse. The resulting offspring is registered in the "appendix" of the American Quarter Horse Association's studbook, hence the nickname. Horses listed in the appendix may be entered in competition, but offspring are not initially eligible for full AQHA registration. If the Appendix horse meets certain conformational criteria and is shown or raced successfully in sanctioned AQHA events, the horse can earn its way from the appendix into the permanent studbook, making its offspring eligible for AQHA registration.