A pen knife is so named because it was used for sharpening a quill

A penknife, or pen knife, is a small folding pocket knife, originally used for cutting or sharpening a quill to make a pen nib. Originally, penknives did not necessarily have folding blades, but resembled a scalpel or wood knife by having a short, fixed blade at the end of a long handle. These knives were used for thinning and pointing quills to prepare them for use as writing instruments and, later, for repairing or re-pointing the quills. They may also be used to whittle wood. Modern penknives may have single or multiple blades and additional tools incorporated into the design.

Over the last hundred years, there has been a proliferation of multi-function knives with multifarious and often gadgets including awls, reamers, scissors, nail files, corkscrews, tweezers, toothpicks, and so on, and the tradition continues with the incorporation of modern devices such as ballpoint pens, LED flashlights, and USB flash drives.

The most famous example of a multi-function knife is the Swiss Army knife, some versions of which number dozens of functions and are really more of a folding multi-tool, incorporating a blade or two, than a penknife with extras.

A larger folding knife, especially one in which the blade locks into place, is often called a claspknife.