A Longsword is a two handed sword, European knights would have used a broadsword or arming sword, or possibly a bastard ( or hand-and-a-half sword) depending on the region.
Longswords are only one type of European sword, and not therefore generic. A far broader and better candidate for universality would be the broadsword. There were many types, the estoc, the hand-and-a- half, the greatsword, but it's not at all correct to associate longswords, double handed swords, montantes, claymores, zweihänders for TWO hands with knights. Knights on horseback preferred lances, spears, sometimes axes or maces, but mainly their knightly sideswords were for 'mano solo' and used as protection, and therefore NOT longswords.