Marshals around the track communicate with drivers using a set of coloured flags, increasingly mirrored by LED light panels and dashboard messages. Each colour carries a specific instruction, and knowing them makes the live timing far easier to read.
Hazard flags
A yellow flag means danger ahead: a car off the track, debris, a slow car. Drivers must slow down and cannot overtake in that sector. A double-waved yellow is more serious: be ready to stop, because the track may be blocked.
A red flag stops the session entirely. Cars return to the pit lane and the clock pauses. During a race a red flag has a strategic twist: teams are allowed to change tyres for free while the cars are stationary, which can hand a lucky driver a cheap pit stop (see safety cars).
A green flag signals the hazard is cleared and racing resumes at full speed.
Blue, white and the warning flags
A blue flag is shown to a slower car about to be lapped by the leaders; it must let the faster car through promptly or risk a penalty. A white flag warns of a slow-moving vehicle on track, such as a recovery truck or a car limping back to the pits.
The black-and-orange flag (with a black circle) tells a specific driver they have car damage and must pit to fix it. A black flag is the most severe: it disqualifies a driver, ordering them back to the pits for good, and is reserved for serious rule breaches (see penalties).
The chequered flag
The chequered flag marks the end of the session. In a race it falls as the leader crosses the line to complete the final lap; every other car takes the flag on the lap they are on when the leader finishes.