Junction buoy

A junction buoy, the IALA preferred-channel mark, stands where a channel divides and shows which of two channels is the main one. It is a modified lateral mark: in Region A a preferred channel to starboard is red with a broad green horizontal band, red can topmark, and a preferred channel to port is green with a red band. The colors reverse in Region B. When lit it shows the composite group flashing Fl(2+1) character, the single trailing flash marking it as a junction rather than a plain lateral mark. The mariner passes it as the lateral mark of the body color to stay in the preferred channel, or on the other side to take the secondary one.