The Yellow Sign appears in Robert W. Chambers’ story of that name in The King in Yellow (1895). Its shape is never drawn for us; we are told only that it is neither Arabic nor Chinese nor part of any script a human hand made. An artist and his model each come into possession of it, and the soft-faced watchman from the churchyard below begins to follow.
The Sign is not a thing you read. It is a thing that keeps whoever reads it. Sight of it opens a line to the King, and no one has recorded a way to cut that line. Absorbed later into the Mythos, it hardened into the mark of Hastur — and whether it summons him or merely recognizes what was already settled has never been separated out.
Because seeing the record and having the thing happen fall in the same place, the Sign stands where our own story keeps standing.