In a five-day Test, if the side batting first leads by at least 200 runs after both first innings, its captain can make the opposition bat again straight away. In shorter first-class matches the threshold is lower, 150 runs for a three- or four-day game.
Why captains sometimes say no
Enforcing the follow-on means the bowlers go straight back out after a long spell in the field. Many captains now prefer to bat again, rest their attack and set a target that removes any chance of defeat, especially on pitches expected to get harder to bat on later in the match.
