Can the course be changed while a race is going on?

Shortening:

• The reasons: the same as for abandoning after the start, and also so that other scheduled races can be sailed.
• The signal: flag S with two sounds, displayed before the first boat crosses the finishing line.
• The new finishing line: at a rounding mark, between the mark and a staff displaying flag S; or a line the course requires boats to cross; or at a gate, between the gate marks.

Changing the next leg:

• How: by moving the next mark or the finishing line and signalling all boats before they begin the leg. The mark need not be in its new place yet.
• A change of direction: flag C with repetitive sounds and the new compass bearing, or a green triangle for a change to starboard, or a red rectangle for a change to port.
• A change of length: flag C with repetitive sounds and a "-" for shorter or a "+" for longer.
• Later legs: may be changed without further signals to keep the shape of the course.

A missing mark: the race committee puts it back or replaces it with a similar one if possible, or replaces it with an object displaying flag M and makes repetitive sounds.

A course is laid for the wind at the start. If the wind swings or dies, the legs stop testing what they were meant to test, or the race cannot finish in time. Moving a mark keeps the race fair without throwing away the part already sailed.