Some days, it feels like the airlines are toying with travelers. The carry-on bag that was fine on Southwest is deemed too big by the JetBlue gate agents. The person in front of you is allowed to put his suitcase in the overhead compartment, while yours – which looks identical – has to be gate-checked before flight. Conforming to carry-on guidelines is maddening, but at least airlines don't put restrictions on the type or style of carry-ons. To them, it's size that really matters.

Types of Luggage Allowed

Airlines don't generally limit the types of bags that passengers may bring as carry-ons. As long as a bag is within the carrier's acceptable size range and doesn't pose an obvious threat to airline staff or passengers, its design doesn't matter to anyone but you.

Carry-on allowances vary by airlines, but most passengers are allowed one carry-on bag and one personal item. A wheeled suitcase is the popular choice as a carry-on because it's easy to transport through the airport. Some travelers opt to carry duffel bags, backpacks or tote bags. Any bag that securely holds all your stuff will do.

The guidelines for what constitutes a personal item are a little more specific. Generally, airlines consider laptop bags, purses, briefcases or diaper bags to be personal items. But again, the size is the most important factor. If you want to use a small backpack or half-filled duffel as a personal item, airline staff probably won't protest as long as it fits under the seat in front of you.

Carry-on Size Restrictions

Like so many things related to flying, the rules about carry-on sizes seem to be constantly shifting. That's because each airline sets it own policies, and they have to do with the dimensions of the carrier's planes. As planes are built and redesigned, their storage space changes too.

Many major airlines including United and Delta allow passengers to use only carry-on bags that are no larger than 9 inches by 14 inches by 22 inches. Southwest accommodates slightly larger bags of up to 10 inches by 16 inches by 24 inches. Alaska Airlines' policies allow for carry-ons up to 10 inches by 17 inches by 24 inches in size. A few international airlines even measure carry-on size in total inches, calculated by adding together the length, height and width of the bag. For example, Air New Zealand allows carry-on bags whose three measurements add up to 46.5 inches total.

There's even more variety in sizing for personal items. Remember, a personal item must be small enough to fit under the seat in front of you. On United, personal items may measure 9 inches by 12 inches by 17 inches at a maximum. Southwest allows 8.5 inches by 13.5 inches by 18.5 inches, while JetBlue's maximum size for a personal item is 8 inches by 13 inches by 17 inches.

If you suspect your carry-on bags are too large, look for a size template in the airport terminal. These guides allow travelers to test their bags against an airline's size allowances. If a bag won't fit into the template, ask a gate agent to help gate-check it.

Choosing Carry-on Luggage

Before buying any carry-on luggage, it's worth checking out the size allowances for each of the airlines you use regularly. Keep in mind that a hard-shell suitcase protects its contents better than a soft-sided bag, but the latter is easier to fit into overhead compartments.

Next, consider how easy or difficult a bag would be to transport through the airport. Bags with wheels are ideal; even travelers who have no issue carrying a heavy bag today might sustain injuries or develop health conditions that make that challenging in the future. Always test luggage before buying it. Wheel or carry a bag around in the store to assess how comfortable the handle is, how smoothly the wheels move and how sturdy the materials seem to be.

Open a carry-on bag to make sure that the interior has plenty of usable space and at least one zippered pocket for securing small items. The bag should also be fairly lightweight when empty. Remember, you'll have to lift it onto the security conveyor belt and into overhead compartments.