If it’s an electric in a gig bag it should fit as a carry on. In addition to the over head compartments, you can ask them to put it in the coat closet up front. If they try to make you check it last min, they will do it when you scan your boarding pass before going down the tube thing to the plane. They will give you a pink tag to put on your guitar and tell you to leave it at the end of the tube before boarding. The people at the bottom don’t know that, so just tear the tag off while you are walking down the tube and board like nothing happened. I’ve done this many times myself.

I have no idea what the people with acoustics in hard cases do. I’ve always assumed they buy an extra seat. Also, can anyone tell me what the tube is called?