Also called balisong or batanga. Not to be mixed with butterfly sword. A knife where the handle is in two parts, and the parts can be rotated around so they enclose the knife to hide the blade. With the right technique, very easy and fast to open. Illegal in many countries, because, like switchblades, they're easy to conceal. Baliswords also exist, often over two feet when opened.
I thought that I had a chance in this fistfight, until suddenly he had a butterfly knife in his hand. Luckily i brought my butterfly sword.
Not to be mixed with the butterfly knife, a butterfly sword is a broad, rougly fore-arm length, single-edged sword, often wielded in pairs. Also has a crossguard to protect the hands, and some "hooks" which can be used to stop or disarm an enemy. Often carried in one scabbard, to hide the fact that you have two of them.
I really want to learn how to use butterfly swords, just like Kano!