14 C.F.R. § 251.3
Small musical instruments as carry-on baggage
Each covered carrier shall permit a passenger to carry a violin, guitar, or other small musical instrument in the aircraft cabin, without charging the passenger a fee in addition to any standard fee that carrier may require for comparable carry-on baggage, if:
(a) The instrument can be stowed safely in a suitable baggage compartment in the aircraft cabin or under a passenger seat, in accordance with the requirements for carriage of carry-on baggage or cargo established by the FAA; and
(b) There is space for such stowage at the time the passenger boards the aircraft.