When are crouch starts and starting blocks compulsory?

World Athletics rules (TR 15.1 and 16.3): in races up to and including 400m (including the first leg of the 4 x 200m, medley relay and 4 x 400m), a crouch start and starting blocks are compulsory, and blocks may not be used in any other race. On their mark, the athlete must be completely within their lane behind the line, with both hands and at least one knee on the ground and both feet on the foot plates, touching neither the line nor the ground in front of it. On "Set" they rise to their final position, keeping hands on the ground and feet on the plates.

Races longer than 400m (TR 16.4): standing start. On their mark, athletes may not touch the ground with their hands, or the line or ground in front of it with their feet.

ASA variation (ASA Rule 29): at club, provincial and associate-member competitions and the mass-participation section of major events, starting blocks are not compulsory. If the organiser makes them compulsory, the organiser must supply them. Athletes' own blocks must meet the World Athletics specifications but can be any design that doesn't interfere with other athletes.