At Gwennap Head, on the south coast of the Penwith peninsula in Cornwall, England, there are a pair of cone-shaped navigation markers that line up with the Runnelstone buoy. They are day markers warning vessels of the hazard of the Runnel Stone. The cone to the seaward side is painted red and the inland one, erected in 1821, is black and white. 

 

Ships at sea know that the black and white marker should always be kept in sight - if it is completely obscured behind the red cone then the vessel is heading for the the submerged rocks nearer the shore.