Guitars and other musical instruments that can fit it the overhead bins are 'cabin legal'. Too much risk for damage to expensive instruments if checked.
Dept. of Transportation final rule:
The rule requires that each U.S. carrier subject to this regulation allow a passenger to carry into the cabin and stow a small musical instrument, such as a violin or a guitar, in a suitable baggage compartment, such as the overhead bin or a closet, or under the seats,