For modern commercial jet aircraft, the "normal" cruising altitude is typically between Flight Level 310 and Flight Level 410, which corresponds to 31,000 to 41,000 feet above sea level. Pilots use "Flight Levels" (FL) to standardize altimeter settings regardless of local atmospheric pressure; for example, FL350 means the plane is flying at a pressure altitude of 35,000 feet. Airlines choose these high altitudes because the air is much thinner, which reduces aerodynamic drag and allows the engines to operate more efficiently, saving significant amounts of fuel. Flying at these levels also keeps the aircraft well above most turbulent weather systems and the majority of general aviation traffic. The specific FL assigned to a flight depends on the "Rule of Hemispheres": generally, flights heading east are assigned odd-numbered Flight Levels (e.g., FL350, FL370), while those heading west take even-numbered levels (e.g., FL340, FL360). This 1,000-foot vertical separation is the primary method used by air traffic control to prevent mid-air collisions in the busy transcontinental and transoceanic skyways.