The 4 × 100 metres relay or sprint relay is an athletics track event run in lanes over one lap of the track with four runners completing 100 metres each. The first runners must begin in the same stagger as for the individual 400 m race. Each runner carries a relay baton. Before 2018, the baton had to be passed within a 20 m changeover box, preceded by a 10-metre acceleration zone. With a rule change effective November 1, 2017, that zone was modified to include the acceleration zone as part of the passing zone, making the entire zone 30 metres in length. The outgoing runner cannot touch the baton until it has entered the zone, and the incoming runner cannot touch it after it has left the zone. The zone is usually marked in yellow, frequently using lines, triangles or chevrons. While the rule book specifies the exact positioning of the marks, the colours and style are only "recommended". While most legacy tracks will still have the older markings, the rule change still uses existing marks. Not all governing body jurisdictions have adopted the rule change.
The transfer of the baton in this race is typically blind. The outgoing runner reaches a straight arm backwards when entering the changeover box or when the incoming runner makes a verbal signal. The outgoing runner does not look back, and it is the responsibility of the incoming runner to thrust the baton into the outstretched hand and not let go until the outgoing runner takes hold of it without crossing the changeover box and stops after the baton is exchanged. Runners on the first and third legs typically run on the inside of the lane with the baton in their right hand, while runners on the second and fourth legs take the baton in their left. Polished handovers can compensate for lack of basic speed to some extent, and disqualification for dropping the baton or failing to transfer it within the box is common, even at the highest level. Relay times are typically 2–3 seconds faster than the sum of best times of individual runners.
