On United Airlines, you can only put your "personal item" under the seat in front of you, provided it does not exceed 17 x 10 x 9 inches (43 x 25 x 22 cm). Your larger "carry-on bag" (max 22 x 14 x 9 inches) must be placed in the overhead bin. If you are traveling on a Basic Economy ticket, you are strictly limited to only one personal item that must fit under the seat; you are not allowed an overhead bin bag unless you are a Premier member or a qualifying cardholder. Items like small backpacks, laptop bags, and purses are perfect for the under-seat space. It is a peer-to-peer best practice to keep essentials like medication, headphones, and snacks in this bag so you don't have to stand up and block the aisle during the flight to access the overhead bins. Just ensure no straps or handles are sticking out into the footwell, as this can be a safety hazard during takeoff and landing.