Common way the elder sign is depicted and used in various Cthulhu mythos related products, which originates from the way August Derleth described it.
If there is one thing I dislike about how the mythos is interpreted in a lot of modern horror games, including Call of Cthulhu and Trail of Cthulhu, it’s the the “Elder Sign”. Generally speaking, it’s treated as an all purpose anti-mythos related protective symbol. When it’s placed on places, objects and people it is supposedly powerful enough to ward away even some of the most powerful of cosmic threats of the mythos. A lot of board games and RPGs use it a general symbol of protection including Elder Sign, Arkham Horror, Trail of Cthulhu, Call of Cthulhu and various others. As it’s so pervasive among Lovecraftian inspired games, it has led to some interesting problems at my own gaming table in Trail of Cthulhu and Call of Cthulhu. This problem occurs because there is an inherent expectation my players have that it is possible to “protect” yourself against mythos entities.
So following is my general description of what the “Elder Sign” was originally, from Lovecraft’s work and others who followed. Then I’m going to get into a personal rant about why I feel there should not be any one sign, symbol or concept that protects against the mythos as a whole.