Slipjoint

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A slipjoint knife is one of the most ubiquitous types of pocketknives. A slipjoint knife consists of a handle with one or more folding blades. These blades are held in position by a strong "backspring" which biases them towards the open and closed position (that is the spring tries to hold the blade closed until it has been pulled past a 90 degree arc from the handle, when the spring force reverses and the blade is inclined to spring to the open position—or at least resists closing once open. Some knife blades feature a "neutral" position where when opened to 90 degrees from the handle, the blade enters a third detent and neither tries to open or close.) Contrast this with the penny knife, which has no locking mechanism other than friction, or locking knives, which mechanically lock the blade in position.