A balisong (AKA a Butterfly Knife) is a folding pocket knife with two counter-rotating handles around the "tang" of the knife. It is prolific because it is easily concealed and can also serve as a multi-purpose weapon. One known user of one is Joshua Montgomery, who carries an 8 inch Benchmade model. How he managed to get a hold of this isn't known, but he likely snuck it in during his early years in the UNSC.