Just came back from an excursion to Nova Scotia (flying within Canada) with the community fiddle orchestra. They let us take fiddles on board as a second piece of carry-on going down, but insisted on checking them on the return loop. Promised to hand-carry them, and then we watched the baggage handlers throwing them all on the conveyor belt into the plane, mostly upside down. OUCHHHH! (No harm done, we checked them all immediately on arrival.)

It was entirely at the airline's discretion whether we could take them on board or not. There were 20 of us so that was a lot of carry-ons.

Note: when we were allowed to carry them on, we weren't allowed to carry any extra strings with the instruments. Extra strings had to go in the checked bags. Ha. Feel safer now?

W-O