Fatto generale: 6840 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 6840 is the number of ways to place 2 non-attacking kings on a 11×11 chessboard.
Trivia: 6840 is a boring number.
Data storica: September 22nd is the day in 1939 that Joint victory parade of Wehrmacht and Red Army in Brest-Litovsk at the end of the Invasion of Poland.