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During the second world war most shipping traffic across the Atlantic travelled in convoys. Merchant ships were formed into a box formation, and were then surrounded by escort ships to protect against submarines. Convoys required careful navigation, as there would often be ships operating only a few ship-lengths to either side. It also required some special equipment. To keep ships from running each other down at night under blacked out conditions they were fitted with a specially shielded blue stern light that could only seen when you were immediately behind the ship. In foggy weather a marker would be towed behind the ship to give the following ship something to follow.  Whenever possible all communications were done visually using flashing light or flag signals by day, and if necessary at night, patterns of colored lights on special signal masts could be used to pass signals through the convoy. In spite of these precautions German U-Boats were very successful in the early years of the war, often taking advantage of their low profile and speed while on the surface to infiltrate convoys at night. In other theaters of the war the lack of a submarine threat often did not justify employing the convoy system, and ships would sail independently.