To legally operate under instrument flight rules (IFR), a pilot can separately add an instrument rating to a private or commercial certificate. An airline transport pilot implicitly holds an instrument rating, so the instrument rating does not appear on an ATP certificate. The FAA issues instrument ratings separately for airplane and powered lift categories and the helicopter class (INSTA and INSTH). Glider and airship pilots may also operate under Instrument Flight Rules under certain circumstances. An individual may hold only one pilot certificate at one time; that certificate may authorize multiple privilege levels distinguished by aircraft category, class or type. For example, an Airline Transport Pilot certificate holder may be permitted to exercise ATP privileges when flying multi-engine land airplanes, but only Commercial Pilot privileges when flying single-engine land airplanes and gliders. Similarly a Commercial Pilot holder with a glider rating may have only Private Pilot privileges for single-engine land airplanes.