The Runway End (Threshold) Identifier is used to indicate to a pilot the positive position of a runway threshold under VFR and non-precision IFR conditions. The REIL contains a master and slave strobe power supply on each side of the threshold. These strobe flash at a rate of approximately twice per second simultaneously. Visual aids required with the use of the REIL include high intensity runway edge lights, runway threshold lights or displaced threshold lights, and runway markings. The PAPI system may also be an associated with a REIL. This system should never be used in conjunction with Pulsed Light Approach Slope Indictors (PLASI) system.