Cabin baggage allowances
Don't you love them? Having done a quick check of the main low-costers, they all appear to have different acceptable measurements for your carry-on bags.
So, if you travel frequently with, say, Easyjet, and follow their online advice on dimensions of your holdall, trolley-dolley wheeled bag, fancy computer bag etc., you'll be fine. There won't be a problem.
But try taking the same bag on Ryanair and you face problems - you'll need a smaller bag unless you want to hear those dreaded words: "Sorry, sir, that bag exceeds the carry-on size - it'll have to be carried as hold baggage. That'll be £40 please."
So, if you travel frequently, use the lowest common denominator - which, as far as carry-on baggage is concerned, is Ryanair.