The primary "altitude rule" in aviation for cruising flight is the Hemispheric Rule (or Semicircular Rule), which helps maintain vertical separation between aircraft flying in different directions. For flights under Visual Flight Rules (VFR) above 3,000 feet AGL: if you are flying an Easterly heading (0°–179°), you must fly at an Odd Thousand-foot altitude plus 500 feet (e.g., 3,500, 5,500, 7,500). If you are flying a Westerly heading (180°–359°), you must fly at an Even Thousand-foot altitude plus 500 feet (e.g., 4,500, 6,500). For Instrument Flight Rules (IFR) flights, the "plus 500" is omitted, meaning Easterly flights are at Odd thousands and Westerly at Even thousands. Additionally, there are Minimum Safe Altitudes: over congested areas (cities), pilots must stay at least 1,000 feet above the highest obstacle within a 2,000-foot horizontal radius. In non-congested areas, the rule is 500 feet above the surface or any person/vessel. These rules are globally standardized by ICAO to ensure that even without air traffic control, planes flying toward each other will always be at different altitudes.