The Auxiliary Studbook, or Hulpstamboek was used by the FPS to help establish the Studbook. It included mares who were of untraceable lineage but who were branded with a F or FS and demonstrated the physical characteristics of a Friesian. The Auxiliary Studbook is now essentially closed since the introduction of tattooing in the 1970′s, and now microchipping which allows the horses’ lineage to be traced.