This page on aa.com (
https://www.aa.com/i18n/travel-info/...on-baggage.jsp) says the following:
Soft-sided garment bags up to 51 inches / 130 cm (length + width + height) can also be taken as your carry-on item.
Musical instruments are also considered a carry-on item and must fit in the overhead bin or under the seat in front of you.
If they meant that it was in addition to your carry on, they would have said "
Soft-sided garment bags up to 51 inches / 130 cm (length + width + height) can also be taken
in addition to your carry-on item." but they did not. now if you'll excuse me, I need to go check my big mac as one of my 3 free checked bags. :P