American Airlines has begun matching ultra-low fares offered by discount carriers Spirit and Frontier on certain routes—particularly routes to popular domestic tourist destinations such as Miami, Los Angeles and Las Vegas. The other big US carriers—United, Delta and Southwest—have started doing so as well, though not as frequently. The fares on the big airlines are up to 50% lower than what you were likely to find before.

Examples: American has offered nonstop round-trip fares as low as $80 from Detroit to Miami…and $86 from ­Chicago to Los Angeles. United has offered a $126 fare from Denver to Miami …and Southwest has offered a $150 fare from Philadelphia to Las Vegas.

These low rates tend to be offered only on routes served by Spirit and/or Frontier and are most likely to be offered for Tuesday, Wednesday and Saturday travel…early morning flights…and during times of year when schools are in session. You can find these fares through fare-booking websites, or if you want to increase the odds that you will hear of them before they sell out, follow Twitter feeds that track airfare bargains, such as my @airfarewatchdog or other feeds such as @theflightdeal or @farecomparedeal.

Bonus: Major airlines tend to tack on fewer or less onerous fees and impose fewer restrictions than discount ones do.

Example: On Spirit, you must pay an additional fee if you want a preassigned seat or the right to stow a carry-on bag in an overhead compartment, among many other potential charges.

But the low airfare from major airlines also may come with limitations. Example: With Delta’s “Basic Economy” fares, passengers don’t get preassigned seats, among other drawbacks. American and United are considering similar steps.