While boats are racing, the race committee may change a leg of the course that begins at a rounding mark, or at a gate by changing the position of the next mark (or the finishing line) and signalling all boats before they begin the leg. The next mark need not be in position at that time.
(a) If the direction of the leg will be changed, the signal shall be the display of flag C with repetitive sounds and one or both of
(1) the new compass bearing,
(2) a green triangle for a change to starboard or a red rectangle for a change to port.
(b) If the length of the leg will be changed, the signal shall be the display of flag C with repetitive sounds and a ‘–’ if the length will be decreased or a ‘+’ if it will be increased.
(c) Subsequent legs may be changed without further signalling to maintain the course shape.