Mercury and High Pressure Sodium lamps may be operated in any burn position and will still maintain their rated performance specifications. Metal Halide and Low Pressure Sodium lamps, however, are optimised for performance in specific burn positions, or may be restricted to certain burn positions for safety reasons. U = Universal burning position; HBU = Horizontal -15º to Base Up; HBD = Horizontal +15º to Base Down; HOR = Horizontal ±15º; H45 = Horizontal to -45º only; VBU = Vertical Base Up ±15º; VBD = Vertical Base Down ±15º. If no special burn position is noted, the burn position is universal.