It would be reasonable to wager that the majority of air passengers have never questioned why the traditional flight altitude is 35,000 feet.
The number has entered aviation lexicon as the catch-all height at which most aircraft travel from A to B. But do all planes fly this high - and if so why?
The higher the better
One of the central reasons behind aircraft altitude is that, as the air gets thinner with every foot climbed, planes can travel more easily and therefore move faster and burn less fuel, saving money.
It would actually be most efficient to be forever climbing, rather than plateau when reaching say, 35,000 feet, cruising altitude, as the weight of the aircraft decreases due to fuel usage and the air thins.
"Each individual aircraft has an optimum altitude (for minimum cost or minimum fuel burn) which will be based on its individual weight," explains Peter Terry, a commercial airline pilot of 30 years.
"Concorde flew at much higher altitudes - 50,000/60,000 feet - where there were no other aircraft and so were able to cruise climb [that is keep climbing]."
Doug Morris, a captain with Air Canada, explains that the general rule is the higher the better "because the thinner air imposes less drag".
"There is a trade-off between fuel efficiency and power," he said.
How's the weather up there?
Flying thousands of feet above the ground also means aircraft avoid much of the bad weather people on the ground are subjected to. You know the feeling when you see nothing but bluebird skies from your window seat, only to descend into your destination airport to dreary drizzle.
The troposphere - that is the atmospheric layer closest to the ground - is home to most of the world's weather phenomenons. Usually measured up to 36,000 feet, this is where clouds are most likely, as well as heavy rains and high winds. Aircraft prefer life in the stratosphere, which means less turbulence.
Avoiding heavy traffic
Flying so high also means that aircraft are able to avoid other airborne traffic, such as light aircraft or helicopters, which fly lower, as well as insects and birds.
Light aircraft do not have pressurised cabins, therefore stick below 10,000 feet. Any higher and the pilot is required to don an oxygen mask to keep conscious.
In the event of an emergency
Should something bad happen to an aircraft at 35,000 feet, like losing power in its engines, the pilot has much longer to deal with the situation, than if the aircraft was just at 10,000 feet. This may sound silly, but remember that planes can still land safely even if both engines fail - so having more time to get your ducks in a row before attempting such a manoeuvre could save lives.