The big push by US based airlines to become more like European budget airlines makes air travel even more confusing.
Depending on the type of ticket you purchase, you may not be able to bring a carry-on to put in the overhead bins at all, unless you are willing to pay extra. So far, every airline still allows you to bring what they call a personal item to stow beneath the seat without charge.
However, each airline has different size limitations. This piece from Smarter Travel has some good information. TIP: If you don’t want to have to click the next button 10 times, look for “View As One Page” beneath the first graphic.
It’s always a good idea to go to the airline’s web page and look at their baggage policies. European airlines generally require smaller carry-on luggage than North American airlines, so If you are looking for new carry-on luggage, look for a suitcase that meets international requirements.