It is a fate you wouldn't wish upon your enemies. It's a form of cruel and unusual punishment. It's a curse that can plague a third of passengers on any given flight.
It's the middle seat, and, technically speaking, it's the worst.
Aside from parents and people flying in groups, few fliers choose to sit here. After you book your ticket and it's time to reserve your seat assignment, you'll find a glut of middle seats lingering before the plane fills up. On Southwest, whose policy makes passengers pick seats after they board, people boarding late are relegated to being sandwiched.
Airplane seats are already too small to be comfortable, so why would you want one wedged between two strangers? Not only are your legs crammed into back of the seat in front, you also typically have body parts of the passengers on either side ramming into you repeatedly.
If you get lucky with good neighbours, there are still the inherent issues of the seat's placement. You're trapped by the aisle-seat passenger, unintentionally restricting your access to the lavatory and overhead compartments. You're in an awkward spot if you want to look out the window. You may know you're taking in the horizon, but to the window-seat passenger, it feels like you're staring right at their head.