Carry-On vs Personal Item
The difference between cabin bags and underseat items, and how both are checked.
Two different limits
Most airlines allow one carry-on and one personal item, but the sizes are not interchangeable. A bag that fits as a personal item will usually fit as a carry-on; the reverse is often false.
Fare classes change the rules
Basic or saver fares may remove the free wheeled carry-on and leave only a personal item. Always read the fare notes on the airline site as well as the size chart.
Related: Airlines · Bags · Sizes · Data reports