What is common understanding for why legal sized writing paper is called foolscap?

The connection is attributable to the former use of a watermark depicting a fool's cap that was used on long sheets of writing or printing paper.

These days, we are most likely to encounter foolscap as a reference to a sheet of paper or, more specifically, to a sheet of paper that is similar in size to a sheet of legal paper.

In the early 17th century, when the use of foolscap was first attested to in English, we would have encountered it as a reference to an actual fool's cap - the cap, often with bells on, worn as part of a jester's motley. How did we get from this colorful cap to a sheet of paper? There are various explanations for the introduction of this watermark-including the claim that a 1648 British parliamentary group substituted it for the royal arms during exceptionally turbulent times-but such explanations remain unsupported by historical facts.