Most airlines allow passengers to bring one carry-on bag plus one personal item onboard for free. Here are the general guidelines from several major airlines:

• Scandinavian Airlines allows one carry-on bag up to 8 kg plus one personal item, with allowance depending on travel class

• United Airlines permits one carry-on bag and one personal item; two backpacks would count as one carry-on and one personal item combined

• Lufthansa allows one carry-on bag (max 55 x 40 x 23 cm) or two items for Business/First Class passengers

• American Airlines permits one carry-on bag plus one personal item, with carry-on size limits of 22 x 14 x 9 inches (56 x 36 x 23 cm)

• Alaska Airlines allows one carry-on bag plus one personal item, with carry-on size limit 22 x 14 x 9 inches (56 x 36 x 23 cm)

• Qatar Airways allows one carry-on bag for Economy (up to 7 kg) and two carry-on pieces for Business and First Class (up to 15 kg total), plus one personal item

• Delta Air Lines allows one carry-on item plus one personal item

In summary, the standard allowance is one carry-on bag plus one personal item per passenger, with size and weight limits varying slightly by airline and travel class.