It doesn’t happen often, but it does happen. Pre-reserved seats are not available when you make your reservations and when you show up for check-in, all that’s left is the dreaded middle seat.
So, ask if you can be seated between two people traveling with the same last name. In all likelihood, they are traveling together and hoping to score the whole row. If you are going to split them up, they are likely to initiate the offer to trade leaving you with an aisle or window seat.
Sweet. From LifeHacker and Tricks of the Trade.
