Conversation so far:
what is the difference between a pen knife and a pocket knife
If a knife has 2 blades and they are pivoted at opposite ends, it is a penknife; if they are pivoted at the same end it is a pocket knife.
what was a pen knife first used for
Originally, penknives were used for thinning and pointing quills to prepare them for use as dip pens and, later, for repairing or re-pointing the nib.
what are pen knives used for today
Today he word penknife is the common British English term for both a pocketknife, which can have single or multiple blades, and for multi-tools, with additional tools.

Question: how long is the blade
Self-contained rewrite: how long is the blade of a penknife