The correct answer would be "sometimes but not necessarily always." The dimensions of cabin baggage may be left up to the airline personnel if the flight is not crowded. But there is no way you would get a guitar on board or through transit at London Heathrow for example. You can try and you may be successful as many report they have been but don't be too surprised if you find it's taken away as checked luggage, either at the counter or at the door of the aircraft. Professional musicians buy a seat for their instruments.
The correct answer would be "sometimes but not necessarily always." The dimensions of cabin baggage may be left up to the airline personnel if the flight is not crowded. But there is no way you would get a guitar on board or through transit at London Heathrow for example. You can try and you may be successful as many report they have been but don't be too surprised if you find it's taken away as checked luggage, either at the counter or at the door of the aircraft. Professional musicians buy a seat for their instruments.