Love it or hate it, flying coach has never been more complicated. Airlines are dividing up the scores of seats behind first or business class into smaller and smaller sections, each with their own set of perks — or lack thereof. The strategy is playing out in the refurbished cabins and new jets of major U.S. airlines. Carriers are nudging passengers to shell out more for comforts like extra legroom, better food and seat-back screens, or perks that used to come with the cost of a ticket, such a coveted aisle seat or one next to your travel partner, or the use of overhead bins. Coach class's roots stretch back to 1950s and '60s when it caught on with carriers that tried to fill more seats with cheaper fares in exchange for a more stripped-down service, since business travelers or other high-paying flyers didn't fill the planes. "Business travelers have always been the bread and butter of aviation," said Bob van der Linden, chairman of the aeronautics department at the Smithsonian Institution's National Air and Space Museum. "They couldn't get a full airplane full of businessmen on every flight." Business and other premium cabins remain an outsize source of revenue for airlines. They accounted for 5.3 percent of international passenger traffic but generated 27 percent of revenue last year, according to the International Air Transport Association, an industry group that represents about 80 percent of the world's airlines. Last week, Delta Air Lines' CEO said the airline is less reliant on coach-cabin revenue as demand for lucrative premium-class seats has surged. But the coach cabin is often the largest piece of the pie, and airlines are paying special attention about how to divide it up as they fit more seats on board and travel demand climbs.

At the high end, there's premium economy on international flights, where passengers are treated to an amenities kit, better dining options and perhaps most important, more legroom and a bigger seat. At the low end, airlines have taken a page from discount carriers' playbook to introduce basic economy, where in exchange for the cheapest fares, passengers can't select a seat in advance, can't make changes to their tickets and have to board the plane last. Executives aren't shy that this is a product they'd prefer passengers to avoid in favor of more expensive standard coach. "It's the only time I've ever seen an industry introduce a product they don't want people to buy," said Henry Harteveldt, a former airline executive and founder of travel consulting firm Atmosphere Research Group. Even in standard coach, passengers can pay up for additional perks like early boarding or lounge access. Low-cost airlines like Spirit are getting into the game, trying to sell their more expensive and roomier seats at the front of the plane. "It's very confusing," Harteveldt said. "The airlines have to be really careful how they name and present these products to the customer." Here is a guide to what the products are and what they give you (and don't).

Basic economy

United Airlines Economy seating Source: United Airlines

Basic economy isn't so much about what you do get as what you don't. It's the same physical seat as in regular coach, but it is also airlines' most restrictive ticket. These passengers board last. They're assigned a seat at check-in. No changes are allowed. All of that is in exchange for what is usually the lowest fare on the plane. U.S. airlines recently expanded these fares internationally, charging travelers for their first checked bag, which used to fly with the cost of the ticket. If you're flying domestically, United won't let basic economy passengers store their luggage in the overhead bins. (American scrapped that requirement this September to better compete with Delta.) JetBlue is planning to launch its own version of basic economy next year.

Main cabin

New seats on JetBlue. Source: JetBlue

This is standard coach. The amount of frequent-flyer miles you earn on these tickets varies depending on how much you pay for your seat. As for on-board perks, these travelers can use overhead bins on the big three U.S. airlines — American, Delta and United — and pick a seat ahead of time for free. That doesn't always mean it will be your ideal location, as we explain in the next section. The fare difference between regular economy class, known as Main Cabin on Delta and American, and basic varies depending on the flight but is often around $50.

Preferred seating

Just because standard economy seats allow passengers to pick a seat ahead of time doesn't mean it will in an ideal spot in the cabin like a window or aisle seat, not a middle seat in row 31. American and Delta have been selling what is known as "preferred seating" for years. United joined them this month. While prices vary, a seat toward the front of the coach cabin, with no extra legroom, can cost an additional $50 to more than $100 for each flight, something to pay attention to if your trip has multiple legs.

Extra legroom

Comfort Plus seating options offered by Delta Airlines Source: Delta Airlines

Delta calls it Comfort+. American's name for it is Main Cabin Extra. United calls it Economy Plus. These more expensive seats have up to six inches of additional legroom compared with standard coach. Passengers also get other perks like earlier boarding, free drinks and dedicated overhead bin space. This ticket can cost an additional $100 on top of the regular economy fare.

Premium economy

American Airlines Premium Seats Source: American Airlines

U.S. airlines are following the lead of a product that international airlines have been offering for years. Premium economy class is often a separate cabin, in between business and economy. Passengers in these seats get an amenities kit, earlier boarding, lounge access, bigger entertainment monitors, better dining options. The seats are wider and offer significantly more legroom and recline compared with regular economy, as well as footrests. The price can be more than double that of a coach ticket. American, Delta and United are scrambling to add this in-between class to their international cabins. American, which is planning on allowing passengers to buy these seats with miles, has been outfitting its Boeing 777s and 787s with these seats, while Delta, which calls it Delta Premium Select, has them on its brand-new Airbus A350s, and has been adding them to its Boeing 777s. United started selling seats in its premium economy section, which it calls Premium Plus, earlier this month for travel starting next March. The cost of one of these tickets varies but is double or more what an economy ticket fetches. For example, a search for a round-trip ticket in mid-April from Newark to Hong Kong in United's Premium Plus showed a price of about $1,450, while regular economy for the same route was about $710.