The items that you're allowed to use as underseat luggage vary between different airlines. Generally, you can use [amazon link="B0762VC1XB" title="small, wheeled suitcases"], small backpacks ...
underseat luggage comes in a variety of styles, including hard- and soft-sided mini suitcases, backpacks, totes, and briefcases. What's more, depending on airline luggage size requirements ...
Most (but not all) Scandinavian Airlines fares allow you to bring one traditional carry-on bag, plus a personal item, which SAS calls an "underseat bag." There's one exception: Passengers ...