This cryptic message means that the light gives a group of three flashes, every 15 seconds, and can be seen in clear weather for 24 miles. By day, mariners can still take visual bearings from lighthouses. By the way, there are rarely more than 60m high, because their light could be lost in clouds in poor weather. Other navigational aids include buoys of two kinds, lateral marks, which mark each side of the channel, and cardinal marks, which relate to compass direction and indicate hazards. These mark each side of a channel and are used when approaching harbours or any narrow strait.
Cardinal marks are yellow and black, and are placed according to the compass direction of a danger.
Each has a different shaped top-mark and light signal. Steering rules are so precise that it is vital to know what other ships sailing in the vicinity are doing and which direction they are heading in. Ships' lights are therefore arranged at specific angles and are displayed to give the experienced navigator all the information he needs.