There are a number* of popular and academic explanations for the invention of the “Jolly Roger” flag with its skull & crossed bones motif.
However this isn’t usually included among them.
The 14th century (originally) church of St. Nicholas in the parish of Deptford on the south bank of the Thames, just upstream from Greenwich, has two seriously imposing and rather ancient stone carvings on its gateposts – skulls with crossed bones.
Local legend has it that these were the inspiration for the “Jolly Roger”.
The claim is based on the activities of privateers – sort of state-licensed pirates – who were key players in the centuries of maritime war between the French, English, Dutch and Spanish. The theory goes that privateers did not want to advertise their nationality as they crept up on an enemy ship, so they flew their own marks of identification, and since many English privateers came from Deptford (where the Royal Navy ship & victualling yards were) they chose a well-known local symbol that would strike fear into the hearts of their victims.
The trouble is, I can’t find reference to many privateers/pirates coming from Deptford – at least, not famous ones. It sounds plausible, but I suspect the basis of the Jolly Roger comes from multiple cultures, designs and historic circumstances that just merged slowly over time.
Also, as the notes in St. Nicholas itself point out, if you look closely at the skulls, they are adorned with laurel leaves, symbolising ‘everlasting life’.
Do you have any knowledge/expertise on this?