Are you getting the best airline prices when you search the internet? Some carriers don't allow some search engines to publish their airfare. Delta and Southwest control over 35 percent of US domestic air travel and may not show up in your search. For example CheapOAir does not list Delta or Southwest flights. Southwest only publishes airfare on it's own site or commercial sites. Delta is sharing with many search engines, but not with CheapOAir, Hipmunk, Hopper, JustFly,Skyscanner, or Trip Advisor. Be careful when you complete your search. If you frequently take Delta or Southwest, you may just want to check their sites.