Once he stopped at a farmhouse well for a cup of water, and all the dogs barked affrightedly at the inconspicuous Zoogs that crept through the grass behind. At another house, where people were stirring, he asked questions about the gods, and whether they danced often upon Lerion; but the farmer and his wife would only make the Elder Sign and tell him the way to Nir and Ulthar. – H.P. Lovecraft, The Dream-Quest of Unknown Kadath
The Elder Sign is a powerful symbol that can be used to ward off paranormal and supernatural creatures, including Outer Gods and Great Old Ones. There are three different elder signs, although any of them are effective against the powers of the Mythos. The symbol is meant to be placed on an opening, doorway, or gateway that will prevent the warded creature from entering. The symbol can be drawn by any means (chalk, carving, lines in dust, pencil, whatever…) and remain effective although if it is created in conjunction with a spell or ritual it is much more powerful. Since the beforementioned manners of making the sign can be easily removed or destoryed, wise users of these potent symbols usually craft them from some other material like lead, stone, iron, or even steel.
A word of caution: the sign is effective only to prevent entrance though entrances like doorways, openings, and gates. Using an Elder Sign in your hand or wearing one around your neck in an attempt to ward off monsters will not work. One cannot simply wander around with an Elder Sign like a crucifix held up to ward off vampires. Additionally, one cannot mark the body with this symbol hoping to avoid possession. These are common misconceptions, and a manner in which many investigators have lost their lives. When faced with evil, flee from it.
A second word of caution: the Elder Sign is a potent symbol of occult power. One should use it wisely as it is not without a cost to one’s soul and stamina.