~ "Flight Level
Used to indicate the altitude at which a aircraft is flying. Each level equals 100 feet. The format for indicating flight level is FLxxx. i.e. FL350 indicates an aircraft will be flying at 35,000 feet. FL's are measured in increments of 10 i.e. you can't fly at FL113, you would have to fly at FL110 or FL120.
There are rules that dictate which FL you can fly at depending on your heading and altitude. These rules are designed to minimize the chance of two planes colliding head on. Aircraft at higher altitudes are given a wider berth because it is assumed that bigger planes like jumbo jets will be flying there.
Between FL180 and FL290 all planes flying at heading 000-179 must fly at odd flight levels i.e. FL190, FL230, while planes flying at heading 180-359 must fly at even flight levels i.e. FL180, FL220. At FL290 and above planes flying at heading 000-179 must fly at every other odd level starting at FL290 i.e. FL290, FL330, while planes flying at heading 180-359 must fly at every other odd level starting at FL310 i.e. FL310, FL350, etc.
All flights above FL180 require IFR