One morning in August I had two Condor A330-900neos cross the screen within a few hours of each other. Same airline, same aircraft type, same patch of sky. D-ANRD, callsign CFG097, was at 39,000 feet. D-ANRN, callsign CFG080, was at 36,000.
Those two numbers are not a coincidence, and the gap between them is not about one aircraft being heavier or newer. One of those aeroplanes was going east and the other was going west, and you can tell which is which from the altitude alone.
The rule
The UK Civil Aviation Authority's table of cruising levels sets it out plainly. An aircraft flying IFR on a magnetic track between 000° and 179° takes an odd flight level — FL250, FL270, FL290. An aircraft on a track between 180° and 359° takes an even one — FL260, FL280, FL300.
Split the compass in half. Everything heading broadly east gets the odd numbers. Everything heading broadly west gets the even ones.
The reason is the obvious one. If opposing traffic is systematically assigned different levels, two aircraft closing on each other at a combined speed north of 1,000 mph are never at the same altitude in the first place. The separation is designed in before anybody has to do anything about it.
So: CFG097 at 39,000 was on an odd level, tracking east. CFG080 at 36,000 was on an even one, tracking west. Same operator, same type, opposite directions, and the altitude line on a small round screen gave it away.
Britain used to do it differently
This is where it gets parochial. For decades the UK did not use the semicircular system at all. It used the quadrantal rule, which chopped the compass into four 90° sectors rather than two 180° halves, and assigned levels accordingly. It was a genuinely British arrangement and it survived a remarkably long time.
It was abolished in 2015, when the UK aligned with the semicircular system used internationally. If you learned your airspace theory before then, you learned a rule that no longer applies — and if you learned it after, there is a decent chance nobody ever mentioned the old one.
Where it does not hold
Worth being straight about the limits, because this is a default rather than a law of physics.
Inside controlled airspace, the level an aircraft is flying is the level air traffic control assigned it. Controllers route around weather, traffic and airspace restrictions constantly, and an aircraft can be stepped to a non-standard level for any number of reasons. You will see exceptions.
The North Atlantic is its own case again — the organised track structure works on a different logic, which is why aircraft out over the ocean do not always look like they are obeying anything you recognise.
And the rule keys off magnetic track, not destination. An aircraft can be broadly heading for somewhere easterly and still be on a track that puts it in the other half of the circle.
Why it is worth knowing
Most of what a tracker tells you is a lookup — a registration, a type, an operator. This is different. It is the one number on the screen that carries information nobody encoded deliberately for you, and it is readable with no reference material at all.
You have read thousands of altitudes. Half the information in them was direction, and it was there the whole time.