Isolated danger marks are erected on or moored on or above isolated dangers of limited extent which have navigable water all round them. On Admiralty charts the position of the danger is the centre of the symbol or sounding indicating the danger.
Black double sphere top marks placed vertically above one another , are a very important feature of Isolated danger marks.
The colours used for isolated danger marks are black with one or more red horizontal bands.
In the case of a buoy a pillar or spar buoy is used.
A white flashing light showing a group of two flashes is used to denote an isolated danger mark.
One or more white bands, letters, numbers or symbols of retroreflective material are used for unlit isolated danger marks in the standard code. One or more pairs of blue above red bands are used in the comprehensive code.