Be aware of carry-on luggage restrictions for a new airline fare. American Airlines and United are offering “basic class” discount fares that prohibit the use of overhead bins. Your carry-on must fit under the seat in front of you…if it doesn’t, you must check it for a fee. Maximum dimensions for under-the-seat carry-ons: American, 18 x 14 x 8 inches…United, 17 x 10 x 9 inches. What to do: Use a duffel or gym bag, which is easy to squeeze into place.

Both airlines began selling this new, ultra-low ticket option recently to compete with discount carriers such as Spirit Airlines and Frontier Airlines, which already offered no-frills fares that meant carry-on bags cost extra.

Basic class fares come with additional restrictions beyond the ban on using overhead bins. The tickets are non-refundable, cannot be changed and are ineligible for upgrades. Passengers are assigned a seat at check-in and are the last to board.

Tips for using basic class fares:

• Always check the price between this fare and the standard economy fare. On some flights, the difference may be as little as $20 and not worth all the restrictions.

• Overhead-bin restrictions are on the honor system. Flight attendants on both airlines are not tasked with policing the use of bins on board the aircraft.

• If you belong to an airline’s premier mileage club or purchase your tickets with a branded (AAdvantage or United Mileage Plus) credit card, you retain your overhead-bin privileges.

• If your carry-on bag doesn’t fit safely under the seat, both airlines require you to check it bag for a $50 fee ($25 for checking a bag, $25 for a gate-handling charge).

• Many name-brand luggage manufacturers now make bags costing under $100 that are specifically designed to fit under aircraft seats. These include Samsonite Wheeled Underseater and Lucas Wheeled Under The Seat Cabin Bag.

Several major airlines do not offer basic Ccass fares, including JetBlue Airways, Southwest Airlines and most foreign-based carriers. Delta Air Lines does, but still allows overhead bin use.