Yes, in most cases, you can choose or change your seat during the online check-in process, which typically opens 24 hours before departure. For many "Main Cabin" or standard fares, this is the first opportunity to select from the remaining seat inventory for free if you didn't pay for a seat at the time of booking. However, if you are traveling on a "Basic Economy" ticket, your seat is usually assigned automatically by the system; while some airlines allow you to pay a fee to change it during check-in, others may lock the assignment entirely. For elite frequent flyers, this window is often when "Complimentary Upgrades" to premium sections like Comfort+ or First Class are finalized. It is highly recommended to check in the minute the window opens to have the widest selection of aisle or window seats. If the online seat map appears full or "blocked," you can still visit the airport counter or gate agent, as they often hold certain seats for operational needs or last-minute adjustments.