But customers will be spared this experience on most flights within the United States, Mr. Aboulafia said, because there just isn’t enough space. “The good news is that pretty much every domestic flight you’re going to take is going to be in a 737 or A320 — no way can you do four-three,” he said.

Traditional airline seats were fashioned out of rigid aluminum frames, then wrapped in thick foam padding. But that approach, said Alex Pozzi, vice president for research and development in interior systems for Rockwell Collins, a manufacturer of aircraft seats, is no longer used, with the availability of more sophisticated, high-tech materials.

“We’ve been using a lot of advanced materials, a lot of composite materials, to allow the actual physical structure to get smaller,” he said. “We’ve also removed a lot of the hard points in the seat and gone to fabric suspension systems,” leading, he said, to seats more akin to ergonomic desk chairs.

Image Rockwell Collins’ Meridian economy seat is the company’s most recent design for single-aisle airliners. The company says the smaller design gives passengers more room.

“The less size that the seat structure itself takes up, the more space that’s left over for the passenger,” Mr. Pozzi said.

Or, as the case may be, for more passengers. “Over the last five years, as slimline seats become more common and were adopted by more airlines, airlines took the opportunity to basically take the space they were saving and, depending on the airline, most of the airlines took that space and added in an extra row or two,” said Jami Counter, vice president of TripAdvisor Flights, which owns the site SeatGuru.com. “The actual pitch would shrink, but theoretically, your leg room wouldn’t.”

“Now,” he added, “you’re cramming another person in there so you still have more people in that exact same space. It becomes a much more unpleasant flying experience.”