I've never tried to take a guitar into the cabin because it never occurred to me to try: those signs in the airport are so very blunt and clear: "If it doesn't fit into this box, you can't take it aboard."

The mandolin (which doesn't exactly fit into the box either) hasn't been turned back yet, possibly because it looks as if it just might if I gave it a good sharp shove. I'm sure it helps that I always ask politely.