Differences in the hardness of fencing blades for epee, foil and sabre

How to recognize the hardness of fencing blades, what causes the hardness and how to choose the hardness of the blade. Every fencer should try different hardnesses and find the suitable hardness for himself as he progresses through his fencing career. Or maybe, like me, have a combination of blade hardness acepeeing to the opponent. But maybe next time :-)