All forms of motorsport use a system of flags or signal lights to inform drivers of hazards during a session. While technological advances has meant that, in Formula One, many flags have been replaced by signal lights, the system of coloured flags has remained constant since the early years of racing.
Single stationary yellow flag means overtaking prohibited due to danger near the track. Single waved yellow flag means overtaking prohibited due to danger on the track. Double waved yellow flag means overtaking prohibited due to blocked track. At both flags, a speed reduction is required as well.
SC board and VSC board
Safety Car is put out on track when an incident blocks the track or leaves a lot of debris. Drivers stay behind the SC until the marshals have cleared up. VSC is for more minor incidents, where no car is on track, but drivers slow down to a required Delta.
Yellow and Red Striped
Slippery track, due to oil, water or loose debris. Can be seen 'rocked' from side-to-side (not waved) to indicate a small animal on track.
Normal racing conditions apply. This is usually shown following a yellow flag to indicate that the hazard has been passed. A green flag is shown at all stations for the lap following the end of a full-course yellow (or safety car). A green flag is also shown at the start of a session.
A blue flag indicates that the driver in front who is one or more laps down must let faster cars behind pass. If the driver ignores consecutive blue flags, a penalty is issued to the driver. Also in free practice and qualifying counts as the fast lap.
Slow moving vehicle ahead - often waved on the last corner during free practice when drivers do practice starts. Also used when an ambulance is on track.
Session suspended or stopped due to imminent danger to competitors, weather is bad or spectators.
Driver is disqualified and must return to pits immediately!!! (usually accompanied by the driver's number).
Black with orange circle
Car is damaged, driver has a mechanical problem and must return to pits as soon as possible (usually accompanied by the driver's number).
Half black/Half white
Warns a driver for unsportsmanlike behaviour. May be followed by a black flag upon further infringement. Accompanied by the driver's number.
Session is completed, but in qualifying you cannot start another lap.