Your carry on luggage has all your clothes and toiletries. Your underseat bag should carry everything you need during your flight so that it's within easy reach.

Personal items are sometimes called underseat luggage because you put them under the seat in front of you when you fly. Since your personal item is already at your seat, you shouldn't have to stand up and open your luggage to get anything during the flight.

Most people bring a backpack as their secondary bag alongside a carry on suitcase or duffel bag.

But some light packers can travel with only a personal item and no carry on bag. That's usually because they're taking a short, weekend trip or because they're flying on a Basic Economy ticket and aren't allowed to bring a carry on.