Southwest Airlines uses a six-character alphanumeric confirmation code (officially called a Passenger Name Record or PNR). One of the most common points of confusion for travelers is distinguishing between the letter "O" and the number "0." To prevent errors and improve legibility, Southwest—along with most major airline reservation systems—strictly uses the letter "O" and does not use the number zero ("0") in their confirmation numbers. This follows a standard practice in the aviation industry to avoid the visual ambiguity between the two characters. So, if you see a round character in your Southwest confirmation email, you can be confident it is the letter "O." Additionally, they generally do not use the number "1" (to avoid confusion with the letter "I"). If you are having trouble logging in to check your flight, ensure you are typing the letter "O." If the code still doesn't work, double-check that you haven't swapped a "5" for an "S" or a "2" for a "Z," though "O" vs "0" is the most frequent culprit.