[dragonwriter]

> I personally use a very large backpack that holds at least as much as a carry-on suitcase, yet the fact that it is in backpack form means they count it as my personal item and don't give me a hard time.

Interesting; they always officially state that the “personal item” must fit under the seat in front of you, which clearly a large backpack won't. No airline's ever enforced it that I've noticed, but then I've never flown Basic Economy (and haven't flown since it was a thing.)

I wouldn't rely heavily, though, on that laxity not changing without notice.

[whistlerbrk]

A full backpacking pack at 60L+ will not. But a stuffed 35L pack will most likely fit.

[just4themoney]

My backpack has no problem fitting under the seat.