Several readers pointed out that actually booking your ideal seat doesn’t mean you’ll get it. Planes are sometimes switched because of maintenance or schedule changes.

Jan Bone, 81, from Palatine, Ill., for example, said she paid extra to sit in United’s Economy Plus section, which offers more legroom, after she had an artificial knee implant. “United switched planes — and voila!” she wrote, “no Economy Plus on the substitute airplane. Fortunately, I had a brief layover in Denver, and the plane for the second half of the ride did have it.” Now, with two artificial knees, she said she is dreading a December trip she plans to make for her granddaughter’s college graduation.

“As a senior needing wheelchair assistance to get to gate and board, the idea of standard economy seating for a 4-hour flight is frightening!” she wrote. “It took me more than a week after that earlier trip to get over the residual pain from the cramped seating.”

Many reader comments focused on the Knee Defender, a set of plastic wedges that slip onto the legs of your tray table and physically prevent the seat in front of you from tilting back.

Those against the Knee Defender contended that the ability to recline your airline seat was an inalienable right and that blocking someone from doing so was downright insolent.

“You have no right to restrict the movement of my seat,” wrote Brian J. Handel, a telephone switch engineer from Little Rock, Ark. “I might add,” he went on, “that if you asked me nicely to not put the seat all the way back I would probably accommodate you. Anything else on your part is just plain rude.”

Those in favor of the device, most of whom pointed out that they were six feet tall or more, recounted how their knees had been repeatedly whacked by passengers who abruptly recline their seat midflight without any warning.