TheauthorfirstreviewstheclassicalKorninequalityanditsproof.FollowingrecentworksofS.Kesavan,P.Ciarlet,Jr.,andtheauthor,itisshownhowtheKorninequalitycanberecoveredbyanentirelydifferentproof.ThisnewproofhingesonappropriateweakversionsoftheclassicalPoincar'eandSaint-Venantlemma.Infine,bothproofsessentiallydependonacruciallemmaofJ.L.Lions,recalledatthebeginningofthispaper.
