I had always thought it was a ''given'' that we read from the dusty old Aviation rules/laws and Air Ticket Conventions, dating back many years (1960, as quoted in the latest Spanish Court ruling) that all passengers could take on board one piece of hand luggage that has to fit under the seat in front of them, plus another small item such as an umbrella or a ladies purse, plus anything purchased in the duty free shops could also be taken on board.
(These are the old definitions - men carry purses today etc) but
AFAIK these laws are still on the air ticket Convention statute books.
This is how all airlines have always defined allowable cabin luggage, until some smart arses in the accounts department started to really mess with everyone's' heads and unravel policy after policy, leading now to chaos, stress and delays on many flights due to the mind boggling, ridiculous, confusing, and now illegal rules surrounding luggage.
It is a total folly.