But on a practical note, some advice for those traveling on these budget airlines: if you pack your carry-on in a backpack rather than a suitcase, every budget airline in Europe and the US that I have flown on will not give you a hard time about it. I personally use a very large backpack that holds at least as much as a carry-on suitcase, yet the fact that it is in backpack form means they count it as my personal item and don't give me a hard time. When I travel I usually have a smaller laptop bag inside a compartment in my backpack, once I get on the airplane I take that out and keep it with me as a personal item (so I can easily access it inflight) and I put my backpack overhead like I would a suitcase. I have successfully done this on Frontier, Allegiant, Spirit, and Ryan Air within the last year and have not had any issues.