A pocket knife is a foldable knife with one or more blades that fit inside the handle that can still fit in a pocket. It is also known as a jackknife or jack-knife. A typical blade length is 5 to 15 centimetres (2 to 6 in). Pocket knives are versatile tools, and may be used for anything from opening an envelope, cutting twine, performing an emergency tracheotomy, slicing a piece of fruit or as a means of self-defense.