In The Dark knight, except for when the Joker had to use big weapons for different reasons, we always saw him using a small knife.
Why does he prefer a knife?
Like we know Batman does not use any gun anytime because it is his own rule. Does Joker follow such a rule? I am not aware of the comic version though! If possible explain it in the scope of the movie, otherwise if there is any special reason according to comics, don't hesitate to share it!