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Companies like Alitalia and KLM show on their web sites hand luggage dimension limits as:

max 55 x 25 x 35 cm

(Equivalent in inches should be: 21.5 x 10 x 13.5 in.)

Would hand luggage measuring 55 x 20 x 40 cm still be accepted as valid?

The sum of all dimensions in both cases is 115 (cm), although the lengths are distributed slightly differently across dimensions.

Some hand luggages like this Roncato measure 55 x 20 x 40 cm (and are classified on the web site as hand luggage, good for cabin).

Generally no. For carry on bags you cannot exceed any individual dimension. That's different for checked luggage where there is restriction on the SUM of the dimensions but not any individual dimension. That's basically a function of where the bags go. Carry on must fit in the overhead bin which is very constrained, checked luggage goes into a bulk container or directly in the cargo hold with much more placement flexibility so it's more about overall volume and weight.

Unfortunately there are no real standards and measuring isn't straight forward either. Most airlines have a test container at the counter. If your carry on looks big to the agent, they will ask you to fit it into the container. If it doesn't go in, you will have to check it. Obviously that's not a test you can run at home or in a store when you buy or order luggage.

For KLMs website: https://www.klm.com/travel/us_en/prepare_for_travel/baggage/baggage_allowance/index.htm

Sizes
* Each suitcase may measure max. 158 cm / 62 in (l + w + h).
** A piece of hand baggage may measure max. 55 x 25 x 35 cm / 21.5 x 10 x 13.5 in (l x b x h).
*** In Business Class you may bring a second, smaller piece of hand baggage that measures max. 45 x 20 x 35 cm / 18 x 8 x 13.5 in (l x w x
h).

Last few times I flew with KLM they had the hard side cases to measure hand luggage, but I have not seen them in use. That was in 2013 and 2014.

It is still up to the check-in and boarding staff to use or not use them.

But when they do, each of the sizes on their site is the maximum. It is not an 'add up' option.

So if they are picky, the 40 cm can be enough to have to gate check your case.

Remember that the rules are more and more restricting and that it is getting more likely over the years that hand luggage sizes are reduced.

Delta airlines has these and it implies that the dimension cannot exceed individual measurements. I highly recommend obeying the size requirements. They might not charge you extra but they might be forced to tag it for storage in the belly of the plane so you would be separated from your luggage.

The reason that carry-ons are generally restricted to such size requirements is due to the fact that it needs to fit into a restricted space such as an overhead bin comfortably enough so that the bin can be properly shut. You wouldn't want to experience some turbulence and have someone's luggage fall on your head, would you?