Racing Rules of Sailing -

http://www.autohoot.com/the_perfect_start.htm#Starting%20sequences

Racing Rules of Sailing - Wikipedia, the free encyclopedia: "Start Signal These signal flags are used in the pre-start procedure. Class flags can be numeral pennants 1 , 2 , and 3 however they can be substituted to avoid confusion with the postponement signals relating to a particular class. When one race signal is displayed over one class flag, the race signal is intended to be read only by that class, and has no effect for the other class. For some classes the class flag is a special, own designed, flag, while for some other classes a flag taken from the International maritime signal flag set are used. The following table shows an example start sequence for Class or Division 3 fleet. Flag Signal Number of sound signals when raised Number of sound signals when lowered Description
3 ↑ Warning Signal. 5 minutes to race start when class flag raised.
3 P ↑ Preparatory signal. 4 minutes to start when P flag raised. Flag P used or if a starting penalty applies I, Z, Black flag or I over Z is used in place of P.
3 P ↓ Long Sound Preparatory signal. P flag removed 1 minute before start. Flag P used or if a starting penalty applies I, Z, Black flag or I over Z is used in place of P.
3 ↓ Start Signal. Race start when class flag removed.
http://en.wikipedia.org/wiki/Racing_Rules_of_Sailing