Many consumers don’t realize that the airfare search apps and online travel agencies they’re using don’t show Delta or Southwest Airlines, which together control over 35% of U.S. domestic air travel. But even some smaller airlines, such as Sun Country, that you may never have heard of don’t show up on major sites. And that can cost you. Recently I saw a $228 round-trip fare nonstop on Sun Country (a very good airline by the way) from Minneapolis to San Juan, where other airlines were charging over $500 for connecting flights.

Check this list in order to be a better informed airfare shopper:

• CheapOAir: No Southwest or Delta
• Expedia: No Southwest, Sun Country Airlines, or Allegiant
• Google Flights: No Southwest
• Hipmunk: No Delta, Southwest, or Allegiant
• Hopper: No Delta or Southwest
• JustFly: No Southwest or Delta
• Kayak: No Southwest
• Orbitz: No Southwest, Sun Country Airlines, or Allegiant
• Priceline: No Southwest or Allegiant
• Skyscanner: Southwest via a travel agency that charges higher prices
• Travelocity: No Southwest, Sun Country, or Allegiant
• TripAdvisor Flights: No Delta or Southwest