Lateral markers are buoys and other markers that designate the boundaries of safe water zones. As you arrive from the open sea or move upstream, green colors, green lights, and odd numbers identify the boundary of a channel on your port (left) side. As you get upstream, the numbers frequently increase. Yellow colors, yellow lights, and even numbers indicate the zone on your starboard (right) side. These colors and markers also help guide large ships through narrow channels. The distance between lateral markers is called their "marker-to-marker width." The minimum recommended marker-to-marker width for most channels in North America is 1,000 feet (300 m). However many channels have been modified over time by dredging or other activities to allow for channels with a marker-to-marker width of 500-600 feet (150-180 m). Large ships need more space than this amount of room to navigate safely. They should avoid crossing paths as far away as possible to prevent accidents caused by close calls with lateral markers. Lateral markers are used not only to define channel edges but also to mark underwater hazards such as rocks, reefs, and sandbars. This allows boaters to select safer routes while avoiding dangerous areas. Lateral markers can be either fixed or floating. Fixed markers are anchored to the bottom and cannot be moved unless removed by federal officials. Floating markers stay within eyesight but can be pushed around by currents or waves.