Several of the stories I’ve written reference the Yellow Sign.  This is something from Chambers’ classic story “The Repairer of Reputations” in The King in Yellow.  The sign is associated with followers of the Hastur’s avatars and features prominently in the Yellow Mythos.

There are several different artistic renderings of the Yellow Sign.  The most widely recognized is the sign developed for the Call of Cthulhu roleplaying game, but other signs have been developed.  A recent one (and my personal favorite) was developed for the King in Yellow roleplaying game. 

The version I’ve developed has a different feel from many of the other versions I could find online, but each symbol is designed to serve different purposes and fits into other styles.