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A plane was diverted mid-flight after two passengers began to argue over a set of knee defenders. But what are knee defenders and why are they so controversial?

A knee defender is a plastic gadget that you clip onto the meal tray of the airline seat in front of you, to prevent the person sitting there from reclining. This is a pretty good explanation of how it works. It's basically the most passive-aggressive product ever created.

The manufacturers have recognised that this may be a problem, so they include a handy “courtesy card” to wordlessly display to your fellow passengers:

“I realize that this may be an inconvenience. If so, I hope you will complain to the airline. Maybe working together we can convince the airlines to provide enough space between rows so that people can recline their seats without banging into other passengers.”

So that's alright then.

Are they even legal?

Surprisingly, the FAA haven't banned knee defenders, but flight attendants will often ask people to take them off as they "interfere with plane equipment". Which is technically true, we suppose. But mostly they really irritate people.

How much leg room do you really get?

Everybody complains about feeling cramped while flying, so SeatGuru.com has a useful seat pitch chart for you to compare legroom on airlines.

It's worth noting that UK regulations stipulate that there should be a minimum of 26 inches between each seat (seat pitch). Here's a useful round-up of the best and worst for seat pitch:

If you fly long-haul in Economy, you might want to avoid China Southern Airways, as it's listed as having just 29 inches of room. British Airways is in the best category, along with a number of other airlines it offers 34 inches of legroom on its planes.

For Premium Economy – or Economy Plus as some airlines call it - Condor Flugdienst and Gol (we've not heard of them either) are stingy on the legroom, with just 31-32 inches. According to SeatGuru, the best is OpenSkies, which has a generous 47 inches of legroom.

If you prefer flying Business, the one to avoid is Czech Airlines as they only give you a paltry 32 inches. The best in this case is Thai Airways with a whopping 87 inches of room.

Are knee defenders necessary?

This question has sparked an emotional debate (on both sides) on airline messageboards, social media and newspapers since knee defenders were invented. It's basically an escalation of the cold war that's always existed between airline passenger and adjacent airline passenger.

Our take? Everybody should be able to recline, as this is a specific function of the seat you buy. Why not try asking politely before you do it though.

However, if you don't mind risking a punch-up with fellow passengers, go right ahead and clip on those knee defenders.

What do you think?