In the late 1990's most major airlines in the world formed a number of airline alliances. The alliances allow the airlines to share in providing priviledges to frequent passengers such as complimentary lounge access and priority treatment at more airports around the world.

Some of the alliances are:

After
http://www.alpa.org/internet/prescorner/hup_1-2000/challenge_2.htm
http://www.latimes.com/travel/insider/19990307/t000020689.html