Everyone knows airplanes' middle seats are the worst. And of course, the airlines realize it, too — and are looking to profit off of our reluctance to be sardined. An increasing number of carriers are reserving window and aisle seats for frequent fliers or passengers willing to shell out an extra $25 or more each way. This makes it harder, or at least more expensive, for families, friends, and couples to sit together. Here, a guide to what could be your next summer travel headache:

Why charge for window seats?
Airlines have become shameless when it comes to wringing extra cash out of customers who want convenience or comfort. Over the last five years, they've started imposing fees for checked baggage, TV viewing, early boarding, and the ability to speed through security checkpoints. "Now they are turning to seats," says The Associated Press.

Which airlines are doing this?
On American, Delta, Frontier, and US Airways, you can pay extra to ensure yourself a window or aisle seat, or one near the front of the plane. Spirit charges an extra $5 to $15 for any advanced seat assignment. Those who don't pay are stuck with the seats they're assigned at check-in.

Can you get a window seat without paying?
Yes, but it's a gamble. You can wait until five days before the flight, when premium seats vacated by frequent fliers upgrading to first class become available at no extra cost. Twenty-four hours before departure, the airlines will release another block of seats set aside for elite fliers or those willing to pay extra. Your final option is to ask gate agents, who often try to help families sit together.

How is this going over with the traveling public?
Not so well. "Who wants to fly like this? It gets more ridiculous every year," Khampha Bouaphanh, a photographer from Texas, tells The Associated Press. Hey, maybe some parents will rejoice if they're forced to sit alone, says Shawna Cohen at Mommyish. "Wouldn't it be nice to tune out for hours, read a magazine, watch a movie, maybe even enjoy a glass of wine? Of course it would be."

Sources: Associated Press, Consumerist, Mommyish