Whether an airline considers a backpack a "carry-on" or a "personal item" depends entirely on its physical size and where it is stored. Most major airlines (like Delta, United, and American) allow one carry-on bag for the overhead bin and one personal item for under the seat. A standard-sized school backpack or a small laptop bag is almost always considered a personal item. However, if you are carrying a large trekking rucksack or a "max-sized" travel backpack (usually over 40-45 liters), it will be classified as a carry-on bag because it cannot fit under the seat in front of you. If you are flying on a "Basic Economy" ticket or with ultra-low-cost carriers like Spirit or Ryanair, the rules are stricter; they may only allow one personal item for free. If your backpack exceeds the dimensions for a personal item (typically around 18 x 14 x 8 inches), you may be forced to pay a gate-check fee. Always check the specific "Personal Item" dimensions for your airline to ensure your backpack qualifies for the free tier.