MasterstrokeMatch play and rules
New ball
A fresh ball is harder, shinier and moves more. Tests offer a second new ball after 80 overs; ODIs start with one from each end.
The new ball is a fresh cricket ball, harder and shinier than a used one, and the best friend a fast bowler has.
A new ball swings in the air because its lacquered surface is smooth, and it seams off the pitch because its stitching is proud. As it wears, both effects fade, the ball goes soft, and batting gets easier. In a Test the fielding side may take a second new ball once 80 overs have been bowled, which is why a captain will often try to have the fast bowlers rested and ready when that moment arrives. In an ODI the innings starts with two new balls, one used from each end, so neither is ever more than 25 overs old. In a T20 a single ball lasts the whole innings.
For a manager the new ball is a resource to allocate. Who opens the bowling, whether to use your best bowler up front or save him, and when to take the second new ball in a Test are all decisions about the same thing: when to spend your bowlers' best conditions.
In Masterstroke: one-day matches use two new balls, and in Tests the ball ages over a wearing pitch, so the timing of a bowling change is part of the plan.