Fait mathématique: 6840 is the number of ways to place 2 non-attacking kings on a 11×11 chessboard.
Culture générale: 6840 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Date historique: September 22nd is the day in 1994 that the Nordhordland Bridge was opened across the Salhusfjorden between Klauvaneset and Flatøy in Hordaland, Norway.