Yes, on American Airlines, most passengers are permitted to bring one carry-on bag and one personal item (such as a backpack) for free. This policy applies even to those traveling on "Basic Economy" fares, which is a major advantage over some competitors. The main carry-on bag must fit in the overhead bin and should not exceed dimensions of 22 x 14 x 9 inches (56 x 36 x 23 cm). The personal item, which would be your backpack, must be able to fit completely under the seat in front of you and should not exceed 18 x 14 x 8 inches (45 x 35 x 20 cm). If your backpack is very large (like a 50L hiking pack), it will be counted as your primary carry-on, and you would not be allowed an additional suitcase. It is important to note that soft-sided garment bags up to 51 inches also count as a carry-on. If the flight is full and overhead bin space runs out, gate agents may ask you to "gate-check" your larger carry-on bag to your final destination, usually for free, but you will always be allowed to keep your backpack with you under the seat for access to your essentials during the flight.