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Although the “Big Three” US carriers operate single-aisle planes — such as the Airbus A320 and Boeing 737 — for most of their popular domestic routes, United Airlines, American Airlines and Delta Air Lines fly twin-aisle jets.

Airlines typically use wide-body jets on long-haul flights, so they usually offer a better inflight experience (with features like seatback TVs, power outlets, and larger overhead bins). But undoubtedly, the best part of this wide-body jet is the ability to reserve premium economy seating for the price of an extra legroom seat.

Here’s what you need to know about getting a “free” upgrade to a premium economy seat on your next flight.

Related: The Complete Guide to Booking Premium Economy Using Points and Miles

How to find ‘free’ premium economy seats

You can reserve premium economy seats on many domestic flights operated by wide-body aircraft, except in a few cases where the airline sells the premium economy seat as a separate cabin (mainly long-haul flights to Hawaii and some transcontinental service operated by Delta). United Economy Plus, American Premium Economy or Delta Comfort+.

That means you can use your complimentary extra legroom seat to reserve premium economy seats – an elite status benefit. If you don’t have elite status you can pay to select these seats. Expect the same perks and service you’d get if you travel in an extra legroom seat, but naturally, a premium economy seat is better.

While booking a flight, check the type of aircraft operating the flight. If you see a wide-body jet like an Airbus A330 or A350, or a Boeing 767, 777 or 787, pull up the seat map.

If you know what to look for, you can tell if the seats are premium economy or not.

For one, the seat configuration will be different from the rest of the economy cabin. On the Boeing 777, as above, the usual premium economy seating configuration is 2-4-2 instead of the standard economy 3-4-3. This is because the premium economy seats are wider.

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Additionally, in the case of United’s Premium Plus cabin, the row numbers of the Premium Economy cabin (rows 20, 21 and 22) are significantly different from the rest of the Economy Plus cabin (rows 30 to 34, 40 and 41). All blue seats including Premium Economy can be reserved as Economy Plus on this plane. That means Premier Silver members are eligible to receive them for free upon check-in. Premier Gold, Premier Platinum, Premier 1K and United Global Services members can select those seats at booking at no charge — and United’s generous Economy Plus companion rules still apply.

American makes it a little harder. You can’t immediately tell from the row number that you’re in premium economy, so pay close attention to the seating layout on the seat map. Premium Economy has a 2-3-2 configuration on this plane, as opposed to the standard 3-3-3 configuration on the rest of the plane.

All orange seats, including Premium Economy, on this flight can be reserved as Main Cabin Extras (American’s extra legroom seats).

Delta Air Lines, which has larger premium economy cabins than United and American, designates some of its domestic premium economy seats as Comfort+. If your Medallion status entitles you to Complimentary Comfort+, you can get one of these seats for free.

If you’re unhappy with the premium economy seats available when you book your flight, or if all premium economy seats are taken, set up an ExpertFlyer seat reminder. (ExpertFlyer is owned by TPG’s parent company, Red Ventures.) If a seat becomes available, you’ll get an email alert. ExpertFlyer has helped me out of a center premium economy seat many times.

Related: How to Use ExpertFlyer Alerts to Get a Better Seat

Premium Economy Seats Vs. Domestic first-class seats

Now, the fun part.

Premium economy seats have significantly better extra legroom than standard seats. Instead, they’re roughly in line with what you’d get from a single-aisle domestic first-class seat.

Let’s compare seat dimensions using United’s fleet. These measurements come United’s website.

As you can see, the width isn’t as spacious as you’d find in first class on a Boeing 737; However, pitch and recline are more than you’d find in narrow-body first class.

So, let’s go back to this example.

For $397, Premier Gold and higher members can reserve a premium economy seat — roughly the equivalent of a first-class seat selling for more than $3,000. Of course, a first-class seat offers decent meals, additional checked baggage allowance, premier access and better service. However, if you look only at the seat, the two are almost in line.

United can change its equipment, possibly moving you to a standard Economy Plus seat, but from my experience, that only happens once in a blue moon.

Related: Why United’s expanded Premium Plus rollout isn’t just good news

Domestic routes with wide-body jets

Here are some domestic flights on which United, American, and Delta typically use wide-body services (at the time of writing). In our tests, you could reserve premium economy seats as extra legroom seats on these routes. As you can see, these are on high-traffic routes to or from hubs. These routes are subject to change.

combined

• San Francisco International Airport (SFO) to Washington, DC’s Dulles International Airport (IAD), Houston’s George Bush Intercontinental Airport (IAH), Chicago’s O’Hare International Airport (ORD), Honolulu’s Daniel K. Inouye International Airport (HNL) and Kahului Airport (OGG) on Maui
• Los Angeles International Airport (LAX) to IAD, ORD, HNL and IAH
• ORD and IAD from Denver International Airport (DEN).
• Newark Liberty International Airport (EWR) to IAH

Note that United also operates multiple daily Premium Economy services from SFO and LAX to EWR that are sold as a separate cabin.

American

• Miami International Airport (MIA) to Dallas Fort Worth International Airport (DFW), Phoenix Sky Harbor International Airport (PHX), Charlotte Douglas International Airport (CLT), Tampa International Airport (TPA), LAX and ORD
• DFW to Orlando International Airport (MCO), Cancun International Airport (CUN), ORD and LAX in Mexico
• CUN and PHX from Philadelphia International Airport (PHL).

Delta

• Hartsfield-Jackson Atlanta International Airport (ATL) to Boston Logan International Airport (BOS), Minneapolis-St. Paul International Airport (MSP), Seattle-Tacoma International Airport (SEA), Detroit Metropolitan Wayne County Airport (DTW), New York City’s John F. Kennedy International Airport (JFK) and LAX
• LAX to DTW

Related: Perfectly good but nothing more: American Airlines’ premium economy across the Atlantic

Bottom line

In addition to a separate cabin at the front of the plane, premium economy seats offer an inflight entertainment system with remote, more generous seat recline, smaller footrests, and (my favorite) tray tables that are wider in the armrests. Just enough to avoid an elbow war with your seatmate.

While your airline can always change its aircraft type, reserving premium economy for the price of extra legroom seats is one of the best domestic flight secrets. The next time you’re looking at a wide-body jet on a flight you’re considering booking, remember that some extra legroom seats may be better than others.