This is a mnemonic I learned a long time ago - maybe even from a crappy thriller novel, now that I think about it - that pilots use to remember what altitude to fly at while flying VFR. It is in everyone's best interests to have a system, so that aircraft going in opposite directions don't end up on collision courses, right? So: when your aircraft is above 3,000 feet MSL and its course (not heading, course) is from 0° to 179° you should fly at altitudes of odd thousands of feet plus 500. When its course is from 180° to 359° you fly at even thousands of feet plus 500. North East odd, South West even - NEODD and SWEVEN. So if you're flying a course of 159° you might fly at 3,500 or 5,500 or 7,500 feet (etc.) and when returning on a course of, say, 339° you'd fly at 4,500, 6,500 or 8,500 feet.