quadrantal heights

The specified flight levels assigned to traffic in each of the four quadrants where such a system is in force. At medium altitudes, odd flight levels are to be flown in the first quadrant (000° to 089°), odd +500 ft in the second quadrant (090° to 179°), even flight levels in the third quadrant (180° to 269°), and even +500 ft in the fourth quadrant (270° to 359°). Above flight level 290, the altitude difference in each of the quadrants is increased to 4000 ft.