When does a foot on a line make the throw a failure?

• The runway: marked by two parallel white lines 50 mm wide and 4 m apart. It is at least 30 m long, and 33.5 m at the top competitions.
• The arc: "The throw shall be made from behind an arc of a circle drawn with a radius of 8m." The arc is a white strip at least 70 mm wide, level with the ground.
• The end lines: a white line at least 0.75 m long runs outwards from each end of the arc, at right angles to the runway lines.
• The failure: in the course of a trial, touching "with any part of their body the lines which mark the runway or the ground outside".
• The arc counts: the notes to the rule name "the throwing arc or lines marking the runway" together, so a touch on the arc is treated like a touch on a side line.
• Loose things: a touch by a loose shoelace or piece of clothing is not a failure. Nor is a touch by something, such as a cap, that was on the athlete at the start and came off.
• Stopping: provided no rule has been broken, the athlete "may interrupt a trial once started, may lay the implement down inside or outside the circle or runway" and may leave the runway. All of it has to fit inside the time for the trial.
• No standing start: unlike the circle throws, the javelin has no rule that the athlete must begin from a stationary position.
• The flags: a judge raises a white flag for a valid throw and a red flag for a failure.

The arc is the line every throw is measured back to, so an athlete who crosses it has thrown from closer than everyone else. The curve keeps the measuring line the same distance from one fixed point, whichever way across the runway the athlete throws.