Can I bring my guitar, in a case, as a carry-on item (on United)?

There’s a size limit for carry-ons. If you have a tiny little guitar that fits within the size limit, then you’ll be ok. If not, I’m guessing no.

Yes, you can. They will require you to gate check it, but it is still far kinder than simply checking it.

Funny you should ask, i just got off a United flight and a guy had a guitar case. As Pixie said, he had to check it plane side but was totally allowed.

I’m surprised, I thought all carry-ons have to fit under the seat.

Certainly not. There are a number of things that you may bring that do not have to fit under the seat—including many rolling bags. If they are too large for under the seat, they should fit in the standard overhead bin. If it is a smaller plane, they will ask you to gate check those as well. Gate checking means they are nicer to the luggage and you get it back faster. There are other special items—like musical instruments—that you may gate check as well.

They understand that you might need to bring something you can’t ship and don’t want to let out of your hands that won’t fit under the seat. They aren’t completely unreasonable. Well, sometimes. If you have an item that is large, but delicate, call ahead and see if they will allow it to be gate checked. They are usually pretty nice about that kind of thing.

I once saw a human sized robot go through airport security. The process was ridiculous and hilarious. However, either it had it’s own ticket (which would not be out of the question) or it was gate checked. No way that thing fit under the seat.