Air traffic controllers direct commercial flights to fly at “even” or “odd” altitude levels. The general rule in most parts of the world is aircraft going East will fly at odd levels, i.e. 35,000 ft., while aircraft heading West will fly even levels, i.e. 34,000 ft. – WTF Fun Facts
Source: Up in the Air: Who Determines a Plane’s Altitude? (thepointsguy.com)