Muted Post Horn

Prominent among these references is the "Trystero symbol", a muted post horn with one loop. Originally derived, supposedly, from the Thurn and Taxis coat of arms, Oedipa finds this symbol first in a bar bathroom, where it decorates a graffitto advertising a group of polyamorists. It later appears among an engineer's doodles, as part of a children's sidewalk jump rope game, amidst Chinese ideograms in a shop window, and in many other places. The post horn (in either original or Trystero versions) appears on the cover art of many TCL49 editions, as well as within artwork created by the novel's fans. Via Wikipedia