About Pocketknives

A pocket knife is a folding knife with a blade that fits inside the handle and that is small enough to fit in a pocket. Blades are typically no larger than 3 to 5 in. (8 to 13 cm) in length. Pocket knives are very versatile tools, and may be used for anything from opening an envelope, to cutting twine, to slicing an apple.
There are many traditional types of folding knives:
A pen knife is a small and often thin knife with one or two pen blades, that does not interfere with the appearance of dress clothes, when carried in a pocket. They were originally intended for making and sharpening quill pens but are versatile and have remained fairly common. Knives with three or four knife blades are also common. A whittler is slightly larger than a pen knife and has three blades. A stockman has a clip, a sheep's foot and a spay blade. They are usually middle-sized. A trapper is large, with a clip and a spay blade. Another traditional styles is the Sodbuster.
Newer multitools have gained popularity in recent years:
The Leathermans and similar combination tools now compete with multi-bladed knives, but most of these are rather large for carrying in a pocket. The "main blade" is typically a pair of pliers and there is typically one non-locking knife blade.
The credit card knife is a very thin knife that is the shape and size of a credit card. It is designed to be carried in the wallet along with regular credit cards. Some of this shape of knife also contain other small tools, such as tweezers, or toothpicks.

Text from Wikipedia