Valid dimensions for hand luggage [duplicate]

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?