Quick Points: Reserve premium economy seats for the price of a coach seat on select wide-body domestic routes

Although the "Big Three" U.S. carriers operate single-aisle planes — like the Airbus A320 and the Boeing 737 — for most of their popular domestic routes, United Airlines, American Airlines and Delta Air Lines fly twin-aisle jets.

Airlines generally use wide-body jets on longer-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 these wide-body jets 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: Complete guide to booking premium economy using points and miles

How to find 'free' premium economy seats

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

That means you can use your complimentary extra legroom seating — an elite status benefit — to reserve premium economy seats. You can pay to select these seats if you don't have elite status. Expect the same benefits and service you'd get if traveling in an extra legroom seat, but naturally, the premium economy seat is better.

When booking a flight, check the aircraft type that operates the flight. Pull up the seat map if you see a wide-body jet like an Airbus A330 or A350, or a Boeing 767, 777 or 787.

If you know what to look for, you can tell if the seats are premium economy.

For one, the seat configuration will differ from the rest of the economy cabin. On Boeing 777s, like the one above, a common 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 those in premium economy, can be reserved on this plane as Economy Plus. That means Premier Silver members are eligible to get them for free at check-in. Premier Gold, Premier Platinum, Premier 1K and United Global Services members can select those seats at booking without charge — and United's generous Economy Plus companion rules still apply.

American makes it a little trickier. 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 those in premium economy, can be reserved as Main Cabin Extra (American's extra legroom seats) on this flight.

Delta Air Lines, which tends to have 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 snag one of these seats free of charge.

If you're unhappy with the premium economy seat available when you book your flight or if all the premium economy seats are taken, set 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 get out of center premium economy seats multiple times.

Related: How to use ExpertFlyer alerts to snag a better seat

Premium economy seats vs. domestic first-class seats

Now, the fun part.

Premium economy seats are significantly better than standard extra legroom 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 from United's website.

As you can see, the width isn't quite as roomy as what you'd find in first class on a Boeing 737; however, the pitch and recline are more than what 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 equivalent to a first-class seat selling for more than $3,000. Of course, the first-class seat offers a proper meal, additional checked luggage allowance, Premier Access and better service. Still, the two are roughly in line if you look solely at the seat.

United could 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 exclusively good news

Domestic routes with wide-body jets

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

United

• San Francisco International Airport (SFO) to Washington, D.C.'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
• Denver International Airport (DEN) to ORD and IAD
• 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) in Mexico, ORD and LAX
• Philadelphia International Airport (PHL) to CUN and PHX

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 fine but nothing more: American Airlines premium economy across the Atlantic

Bottom line

In addition to a separate cabin closer to the front of the plane, premium economy seats tend to offer an inflight entertainment system with a remote, a more generous seat recline, a small footrest and (my favorite) a tray table in the armrest that's wide 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 an extra legroom seat is one of the best domestic flight secrets. The next time you see a wide-body jet on a flight you're considering booking, remember that some extra legroom seats could be much better than others.