Not long ago I got a question from a fellow who was pretty upset about an unexpected fee. He asked, "When the hell did airlines start charging for seats?"

My answer: Not all airlines do this, but some have been charging extra for seats for years and more are joining the party. There are ways to avoid this fee but the first thing you need to do is understand what the fee is and isn't.

When we buy a plane ticket in economy class, most of us assume we're buying a seat. True, and we will get "a" seat, but it may not be the one we want. I'm not talking about more legroom; by now we know we must pay a premium for those. This fee for most-wanted seats concerns standard aisle seats and window seats.

Airlines have figured out there's a demand for seats next to the aisles and windows, and whenever there's a demand for something -- whether for checked-bags or blankets -- airlines slap a fee on it, and that's what they're doing for seats. Actually, it's not a fee for aisles and windows, per se; it's a "seat selection" fee. You don't pay for a specific seat but for the privilege of choosing a seat ahead of other passengers. Call it a cut-in-line fee and it doesn't have to cost much; I've seen such fees advertised for $6 and up. It may be worth it if you must have a particular seat or want to be sure of sitting with family or friends.

Like all fees, it's not mandatory and as I said, many airlines don't charge for seat selection. But some of those that don't charge, do have a catch: Spirit Airlines will randomly assign you a seat for free and ditto for Frontier. Others, like Lufthansa, let you choose yourself but not until the final 24 hours before take-off. Either way, the odds of sitting with your travel companions is not very good while your chances of getting stuck in a middle seat are vastly improved.

By the way, not all airlines that have seat selection fees use it for all flights. For example, there is no seat selection fee if you fly Aer Lingus from New York to Dublin but you will pay for flying Dublin to London.

Sometimes a seat selection fee is not really a seat selection fee and Delta is a case in point. On its Basic Economy fares, Delta offers no seat selection at all; the airline assigns the seats and that's that. If you want to choose, you must bump up to the more expensive Main Cabin. By the way, American Airlines is said to be mulling over a basic class of its own and it would not surprise me at all if a seat selection goes out the window on that plan.

As for Southwest, it doesn't charge a seat selection fee either, yet the airline's open seating plan means the first passengers on the plane get dibs on aisles and windows; if you want to improve your position and have a better shot at better seats, you can pay the $12.50 early boarding fee.

Do airlines really need to charge another fee? As we saw in 2015, four of them (American, Delta, Southwest and United) made billions in 2015. Fees were a big part of that, along with lower oil prices. So why are fares still up there and fees being charged even as prices at the pump come down? The answer is demand for air travel remains steady. Airlines don't have to lower fares and don't have to remove fees, not as long as people are willing to pay.

Does your airline have a seat selection fee? Find out on its website. If the fee is included and you don't want to pay, you have a couple of options: resign yourself to playing seat-roulette and take whatever the airline gives you. On an hour-long flight, a middle seat can be bearable. Or, if you have the option of choosing a seat within 24 hours before take-off, set an alarm for precisely 23 hours and 59 minutes before departure and check to see if any good ones are left. You might get lucky.