When the ball is changed, and who gets to decide

The ball as regulated equipment

A cricket ball is manufactured to a specified weight and circumference and is expected to change during use in a broadly predictable way. That expected change is part of the contest, since a hard new ball behaves differently from a soft old one and each state rewards a different skill. The laws therefore treat wear as legitimate and alteration as an offence, which obliges umpires to distinguish between the two by inspection alone.

Because the difference is one of degree rather than of kind, the judgement is inherently harder than most decisions an umpire is asked to make. Everything about replacement follows from that tension between the wear the game wants and the interference it forbids.

Losing shape, and the replacement gauge

A ball that has gone out of shape no longer behaves as designed, and either side may ask the umpires to inspect it between deliveries. The test is mechanical rather than impressionistic, using a gauge through which a ball of legal dimensions should pass without forcing. A ball that fails is replaced with one of similar age and condition rather than with a new one, so that neither side receives a windfall.

Judging similar condition is a matter of experience, and the chosen replacement rarely satisfies both captains at the same time. The provision exists to preserve the state of the contest rather than to reset it, which is why a visibly newer ball draws such strong objection.

Lost balls and damage away from the bat

A ball hit out of the ground or otherwise lost is replaced on the same principle of matching age and condition as closely as the available stock allows. Grounds keep a graded box of used balls for exactly this purpose, which is why a replacement can usually be produced within a minute or two. Where the ball has been damaged by contact with an advertising board or a hard surface, the same matching approach applies.

The fielding side has no entitlement to a new ball in these circumstances, however much wear the previous one had accumulated. That constraint is what stops a convenient loss from becoming a tactical route to fresher equipment.

The new ball a side is entitled to

In the long format the fielding side may take a new ball after a set number of overs, and this is a genuine entitlement rather than a discretion. Choosing when to take it is a real decision, because a new ball offers hardness and seam movement while surrendering whatever reverse movement the old one had developed. A captain with tiring quick bowlers and effective spinners may defer, since the new ball suits one of those groups considerably more than the other.

Conditions matter too, and taking it under lights or in heavy air is a different proposition from taking it in dry afternoon sunshine. The entitlement recurs at intervals, so the same decision returns several times across a long innings.

Alteration, and why it is policed as it is

The laws permit polishing on clothing and drying with a towel while forbidding artificial substances and any deliberate roughening of the surface. The line falls there because polishing accelerates a change the ball would undergo anyway, whereas roughening produces an asymmetry it never would. Enforcement depends heavily on inspection and on what officials happen to observe, which makes this one of the least evenly applied areas of the laws.

Sanctions typically combine a penalty score, a change of ball and a disciplinary process that sits outside the match itself. Debate about whether the line is drawn in the right place recurs, and it turns on how much lateral movement the game wants to make available to bowlers.