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The Elder Sign is an icon, symbol, or rune in the Cthulhu Mythos, whose stories describe it as a form of protection against evil forces. Although not described in Lovecraft’s work, he illustrated it in correspondence as a line with five branches.
Lovecraft described the Elder Sign only once in his writings, as given by the aged alcoholic Zadok Allen in “The Shadow Over Innsmouth” (1936): “In some places they was little stones strewed about — like charms — with somethin’ on ’em like what ye call a swastika nowadays. Prob’ly them was the Old Ones’ signs.” In this story, the sign is used as a defense against Deep Ones; apparently, the Deep Ones cannot harm someone protected by an Elder Sign. However, Lovecraft is known to have drawn it in at least one of his correspondences as a single line with 5 shorter lines branching off. Mythos writer August Derleth described the Sign as a warped, five-pointed star with a flaming pillar (or eye) in its center, and it is this interpretation which has become the most popular in subsequent Mythos literature.
Alchemical symbols, originally devised as part of alchemy, were used to denote some elements and some compounds until the 18th century. Although notation like this was mostly standardized, style and symbol varied between alchemists, so this image above lists the most common.