In the U.S., a survey disk is usually engraved with the name of the agency that placed it. The name of the mark (or the "station" it locates) is usually stamped into its surface, along with the date on which it was set (or re-set, since markers that have been destroyed can be replaced). Disks marked with a triangle are known as "triangulation station disks", since they mark the position of the primary point used for triangulation (or map-making). Disks marked with an arrow are called reference marks (RMs), since they point (or "refer") to the principal station that may be located many feet away. A triangulation station often had two or three reference marks. Reference marks were set to enable the primary station to be re-established (or re-set) if needed.