H NatRose ANYTHING can be a carryon as long as it's no larger than 10X16X24 (box, backpack, soft or hard suitcase, or even an item in a plastic bag. Your carry on should be small enough to fit under the seat in front of you. As a practical matter, if you exceed the above dimensions by an inch you'll be fine. Some airlines stick strictly to their requirements. I've never seen Southwest do that. Just don't be outrgeously too large.
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