Luggage must be stored underneath the seat in front of you and must fit and not be sprawling out into the space between seats or into the aisles, according to FAA regulations.
Knives, scissors, guns and firearms are not allowed in any of your under-seat or overhead bags. Store these items in your checked luggage if you must transport them.
Most airlines require your carry-on baggage to fit into a 22-inch by 14-inch by 9-inch rack, but the space underneath the seat in front of you is notably smaller. You might want to impose your own restriction of a soft-sided bag if you plan to put it underneath a seat so that it can bend more easily into the space provided.
