A butterfly knife has a blade concealed between two halves of the handle that can be brought to bear quickly.
Benefit: If you are proficient with the butterfly knife (or have the Quick Draw feat) and are holding it in your hand, you may open it as a free action; a nonproficient user must spend a move action to open it. Otherwise, treat this weapon as a dagger. When it’s closed, you gain a +2 bonus on Sleight of Hand checks made to conceal it on your body.
Pathfinder Companion: Adventurer’s Armory. Copyright 2010, Paizo Publishing, LLC; Authors: Jonathan Keith, Hal Maclean, Jeff Quick, Christopher Self, JD Wiker, and Keri Wiker.