The original difference between a jackknife and a penknife was size.
Both had blades that folded into the handle for safety.
The small penknife came first and was carried in a pocket in a sheath and was used for making or repairing quill pens.
Pen is derived from penna, the Latin word meaning “feather.”
The jackknife was simply a large, all-purpose penknife, so called because it was a handy tool for sailors, who, at the time, were called “Jacks.”