The Sign of Thereamid is a set of three straight lines and one curved one that is always found in or near a boundary point between a world and the Between.
The Fae have long been aware of the significance of the Sign of Thereamid. It existed before they knew of it, and they have strict taboos about imitating or altering it, for fear of angering whatever being created the marks.
The original document is available at http://www.thereamid.com/tiki/Thereamid+Sign