Members of the public are being encouraged to help record strange carvings known as "witches' marks" which can be found in medieval houses, churches, and barns.
The markings, which have been found in places like the house where Shakespeare was born and the Tower of London, are ritual protection symbols from a period when belief in the supernatural was common.
But the symbols have never been comprehensively recorded, and this Halloween, government heritage agency Historic England is calling on the public to share information and photos of any they know about.
The marks, also known as apotropaic marks, were carved on to wood or stone near entrances like doorways and windows, to protect inhabitants and visitors from witches and evil spirits.
Markings range from a "daisy wheel", which looks like a flower drawn with a compass, to a single endless line which was supposed to confuse and trap evil spirits, to "Solomon's knots" and pentacles.
Sometimes they take the form of the initials AM for Ave Maria, M for Mary or VV for Virgin of Virgins.
Duncan Wilson, chief executive of Historic England, said: "Witches' marks are a physical reminder of how our ancestors saw the world. They really fire the imagination and can teach us about previously-held beliefs and common rituals.
"They were such a common part of everyday life that they were unremarkable and because they are easy to overlook, the recorded evidence we hold about where they appear and what form they take is thin.
"We now need the public's help to create a fuller record of them and better understand them."