Has it ever crossed your mind why some people have excessively large bags tucked beneath their seats during flights? It's quite unclear how baggage policies work, but as a general rule, personal items should fit comfortably under the seats, because if it's too large, it should be considered as a carry-on and must be stored in the overhead bins.

In these times, where low-cost flying is rampant and that carry-ons are charged extra, more travelers would now be forced to carry only a personal item. The differences between a personal item and a carry-on are quite unclear for most people, so how do airlines really determine which is which?

Now, if you get lucky, you might get away with a large bag tucked underneath your seat. However, it's important to know a few basic rules so you'd know how to pack for your next travels. Listed here are a few guidelines on how some of the major U.S. airlines determine carry-ons and personal items.

American Airlines. Personal items must have a dimension of 18x14x8 inches and anything that exceeds that measurement is considered as a carry-on. According to Seat Guru, carry-ons must be or under 22x14x9 inches.

Delta. The airline doesn't really have specified dimensions for personal items. According to Smarter Travel, as long as your things fit underneath the seats in front of you, you're good to go. But for its carry-on policy, your luggage should be 22x14x9 inches at the most.

United. For personal items, it must have a measurement of not more than 17x10x9 inches. Meanwhile, carry-ons should not exceed 22x14x9 inches.

Southwest. Southwest doesn't also have specifics about its personal item policies, but it does count pets as either a personal item or a carry-on. For its carry-ons, luggage must not exceed 24x16x10 inches.

JetBlue. Personal items must be or under 17x13x8 inches and should be small enough to fit under the seats. For carry-ons, it should be 22x14x9 inches at maximum.

Now, whenever you travel, make sure that you measure your items first. You don't want to incur additional fees along the way.