Your guide to the new 'basic economy' airfares

In an attempt to level the playing field with ultra-low-cost airlines Allegiant, Frontier and Spirit, three of the four remaining "legacy" airlines (American, Delta and United) have begun selling less expensive "basic economy" airfares. So now you'll have as many as five airfare products to choose from on some flights: first class, business class, premium economy, regular economy and basic economy.

The two most important features of basic economy to note: On some airlines, you're not allowed to place a bag in the overhead bins (whatever you carry onboard must be small enough to fit safely under the seat in front of you). And, because you can't choose a seat in advance, you'll be assigned a middle seat at check-in or at the gate, probably at the back of the plane.

But there are other restrictions and rules to note, which we've broken down here by airline, and there are subtle differences between airlines. For example, Delta customers earn full frequent-flier accruals and can use overhead bin space, but not those on American and United.

Southwest, Alaska, JetBlue, Virgin and most foreign-based airlines haven't introduced this new fare class. And while Frontier still allows changes and cancellations to basic-economy tickets for a fee, American, Delta and United do not.

When buying these fares, note that on shorter routes, the price difference between basic economy and regular economy may not be worth the perks you surrender, so choose carefully.

What it's called: Basic economy.

Maximum carry-on bag size: 22 by 14 by 9 inches.

Overhead bin use? Allowed.

Advanced seat selection: After check-in or at gate only.

Upgrade eligible? No.

Fee to change/cancel? No changes or cancellations unless made both within 24 hours of purchase and 7 days ahead of travel.

Boarding priority: None, even if you have frequent flier status.

Exemptions: None.

Award miles and elite qualification: All SkyMiles members are eligible for mileage accrual and will earn Medallion Qualification Miles, Medallion Qualification Segments and Medallion Qualification Dollars on Basic Economy fares.

What it's called: Basic economy.

Maximum carry-on bag size: 18 by 14 by 8 inches. Must fit under the seat in front of you (larger items brought to the gate incur a checked bag fee plus a $25 service fee).

Overhead bin use? Not allowed (see exemptions).

Advanced seat selection: You can choose a specific seat within 48 hours of departure, but a fee will apply. Otherwise, seats will be automatically assigned for free when you check in; however, if you enter your children's age in the reservation they will try to seat those aged 13 and under with parents.

Upgrade eligible? No.

Fee to change/cancel? No changes or cancellations unless made both within 24 hours of purchase and 7 days ahead of travel.

Boarding priority: Last to board.

Exemptions: AAdvantage members with elite status and AA credit card holders keep all boarding and overhead bin privileges.

Award miles and elite qualification: Elite Qualifying Miles and Elite Qualifying Segments are earned at a reduced rate of 0.5 per mile/flight segment flown, regardless of frequent flier status.

What it's called: Bare Fare.

Maximum carry-on bag size: Free bag must measure 16 by 14 by 12 inches including handles and wheels, and must fit under seat. Paid carry-on size is 22 by 18 by 10 inches.

Overhead bin use? Allowed, for the above fee.

Advanced seat selection: $1 to $50.

Upgrade eligible? Yes (to "Big Front Seat" for $25-$75 per flight segment).

Fee to change/cancel? Not allowed, unless made within 24 hours of booking and 7 days ahead of travel.

Boarding priority: Not applicable.

Exemptions: None.

Award miles and elite qualification: Standard and/or not applicable.

What it's called: Economy class.

Maximum carry-on bag size: 24 by 16 by 10 inches (but under 35 pounds).

Carry-on bag fee: $30-$35 each way if paid for on FlyFrontier.com at booking and up to 24 hours before the flight. $60 at the departure gate.

Overhead bin use? Allowed, upon payment of a carry-on bag fee.

Advanced seat selection: Allowed for $6 via FlyFrontier.com, at booking and up to 24 hours before departure, or free at check-in or at gate.

Upgrade eligible? No.

Fee to change/cancel? $99 ($75 to change the name of a passenger).

Boarding priority: None.

Exemptions: None.

Award miles and elite qualification: Standard and/or not applicable.

What it's called: Basic economy.

Maximum carry-on bag size: 9 by 10 by 17 inches; must fit under seat.

Overhead bin use? Not allowed. Everyone else who brings a full-sized carry-on bag to the gate will be required to check their bag and pay the applicable checked bag fee plus a $25 gate handling charge.

Advanced seat selection: Not allowed. Assigned at boarding. There is no accommodation for families traveling together.

Upgrade eligible? No.

Fee to change/cancel? Not allowed (unless made within 24 hours of booking and 7 days ahead of travel).

Boarding priority: No.

Exemptions: MileagePlus Premier member or companion traveling on the same reservation, the primary card member of a qualifying Mileage­Plus credit card, or a Star Alliance Gold member may use overhead bins.

Award miles and elite qualification: These fares do not earn Premier qualifying dollars, Premier qualifying miles, Premier qualifying segments, Lifetime miles or credit toward the four-segment minimum.