There were about 725 airlines in the world in 2012 providing different range of services. Air transport have a freedom of route choice than most other modes.The mode is less restricted than land transport to specific rights of way, nevertheless much more constrained than what might be supposed. Early in the history of aviation, physical obstacles such as the rocky mountains and the great gap.
The limitations that structure air transportation are mainly human creations. First in the interest of air safety, air traffic is channeled along specific corridors so that only a relatively small portion the sky is in use.