Airlines often define exactly what size and weight a carry-on bag can be, but then also allow you to bring a 'personal item' (a purse or laptop bag) of often undefined size. Sometimes airlines give specifications, but these vary, and often the advice is for it to 'fit under the seat in front of you'. I for one want to pack as much as I can without causing trouble for others, which is pretty hard to achieve with the information I have. So is the space under a seat standardized across planes?
Browsing this site and elsewhere, I can find exact specifications for some airlines, but that doesn't help much when your airline (as is my case) says:
You can also bring one small personal item on board. This could be a little handbag or a slim laptop case that fits comfortably under the seat in front of you.
Make sure you don't overpack!
Bulky hand bags, shopping bags or large laptop cases may not qualify as a small personal item. If you have too much baggage, you'll have to pay to check it in.
In addition to [carry-on] allowance, all passengers are allowed to bring a handbag or small laptop bag onboard for free.
I'm hoping that if this question is answerable it would help more than the people who happen to use these airlines.