The purpose of hemispheric altitude rules
Hemispheric cruising altitude rules exist for one reason: vertical separation. By assigning different altitudes to aircraft flying in roughly opposite directions, the system reduces the likelihood of head-on or near-head-on conflicts at cruise altitude. Two aircraft approaching each other on reciprocal courses at the same altitude have a combined closure rate that can exceed 300 knots in GA — leaving almost no time for see-and-avoid to work.
The concept is simple: aircraft heading generally eastbound fly at different altitudes than aircraft heading generally westbound. This provides at least 500 feet of vertical separation between opposing traffic flows in VFR and 1,000 feet in IFR.
FAR 91.159 — VFR cruising altitudes
14 CFR 91.159 applies to VFR flights above 3,000 feet AGL in uncontrolled airspace (Class G) or when not assigned an altitude by ATC. The rule divides the compass into two halves based on magnetic course:
Magnetic course 0 through 179 degrees: Fly at odd thousands plus 500 feet MSL. Examples: 3,500; 5,500; 7,500; 9,500; 11,500.
Magnetic course 180 through 359 degrees: Fly at even thousands plus 500 feet MSL. Examples: 4,500; 6,500; 8,500; 10,500; 12,500.
The +500 foot offset is what separates VFR traffic from IFR traffic at the same hemisphere. An IFR aircraft heading east at 7,000 feet has a VFR aircraft heading the same direction at 7,500 feet — 500 feet of separation. VFR traffic heading the opposite direction is at 8,500 feet — 1,500 feet below the next IFR westbound altitude of 8,000.
Critical detail: the rule says "magnetic course," not magnetic heading. Course is the intended path over the ground before wind correction. Your heading may differ significantly from your course due to crosswind, but the altitude selection is based on the course line drawn on the chart.
FAR 91.179 — IFR cruising altitudes
14 CFR 91.179 governs IFR altitude assignments below Flight Level 290. The hemispheric rule is the same directional logic but without the 500-foot VFR offset:
Magnetic course 0 through 179 degrees: Odd thousands MSL. Examples: 3,000; 5,000; 7,000; 9,000.
Magnetic course 180 through 359 degrees: Even thousands MSL. Examples: 4,000; 6,000; 8,000; 10,000.
In practice, ATC assigns IFR altitudes and will generally follow the hemispheric rule unless traffic or airspace constraints require a different altitude. Pilots filing IFR should request an altitude consistent with the hemispheric rule for their direction of flight. If ATC assigns a non-standard altitude, comply with the clearance — ATC is providing the separation.
Above FL290, Reduced Vertical Separation Minimum (RVSM) airspace uses 1,000-foot separation and a different altitude table. This primarily affects turbine aircraft and is outside the scope of most GA operations.
The NEODD SWEVEN mnemonic
The most widely used memory aid for hemispheric altitudes is NEODD SWEVEN:
NE-ODD: Northeastbound (magnetic courses 0 through 179 degrees) = Odd altitudes.
SW-EVEN: Southwestbound (magnetic courses 180 through 359 degrees) = Even altitudes.
For VFR, add 500 feet to the IFR altitude. So NEODD becomes "odd + 500" and SWEVEN becomes "even + 500."
Some pilots prefer the simpler "Odd + 500 heading east, even + 500 heading west" or visualize a compass where everything on the right side (east) is odd. Whatever method works — the key is consistency. Pick a mnemonic, internalize it, and never fly at the wrong altitude for your direction of travel.
When the rules apply and exceptions
Understanding when these rules do and do not apply prevents both violations and confusion:
VFR (91.159) applies: When flying above 3,000 feet AGL, in level cruising flight, and not assigned an altitude by ATC. Below 3,000 feet AGL, pilots may fly at any altitude appropriate for the terrain and traffic pattern.
VFR (91.159) does not apply: When flying VFR in Class B, C, or D airspace and assigned an altitude by ATC. When receiving VFR flight following, ATC may suggest an altitude for traffic separation — this is advisory, not mandatory, unless you are in airspace that requires ATC altitude assignment.
IFR (91.179) applies: Below FL290 in uncontrolled airspace, and as the baseline for ATC altitude assignments in controlled airspace.
IFR (91.179) exceptions: ATC may assign any altitude necessary for traffic separation, regardless of hemispheric rules. When ATC assigns an altitude, that clearance supersedes 91.179. The pilot should comply with the clearance and may request a different altitude if operational needs require it.
3,000-foot AGL rule: Note that this is AGL (above ground level), not MSL. Over mountainous terrain at 8,000 feet MSL, 3,000 feet AGL might be 11,000 feet MSL. Below that, hemispheric rules do not apply to VFR traffic.