Looking for a little extra space in economy without paying for an upgrade? Many frequent travelers use what's known as the 3-seat economy trick.

The strategy is simple: if you're traveling with a companion, book the window and aisle seats in the same row while leaving the middle seat unoccupied. The idea is that most passengers prefer window or aisle seats, so the middle seat may remain empty if the flight isn't full.

When it works, both travelers get extra personal space, making long flights more comfortable without spending more on premium seating.

However, there's no guarantee. Airlines are filling more seats than ever, and a last-minute booking, standby passenger, or seat reassignment can easily place someone in the middle seat. On full flights, you'll simply end up sitting on opposite sides of another passenger.

If seat selection is included with your ticket, there's little downside to trying the strategy. It tends to work best on flights during off-peak travel periods when there are more empty seats available.

The Takeaway: The 3-seat economy trick is a free seating strategy that can occasionally give you extra space, but it's far from guaranteed. Consider it a bonus if it works—not something to rely on for every flight.