Originally Posted by
highpeaklad
i thought it was a direction thing.
East to west odd numbers, west to east even.
North to south even, south to north odd.
Dont know if it still holds true but it was in a book when I was young.
What you refer to is cruising Flight Levels, I believe, though they are not the same everywhere. It‘d be inverted actually. Eastbound odd, westbound even.