Roller, rolling

The roller - and there might be more than one, of different weights - is the device that the groundsman will use to roll and flatten the pitch before play commences.

If the start of the match is delayed for any reason, the batting captain may ask for the pitch to be rolled again before play finally commences. The umpires will agree to this only if the delay has caused a significant effect on the state of the pitch since it was originally rolled - through heavy rain, for example. Once the match has started, the captain of the batting side may request that the pitch be rolled before the start of each subsequent day's play and before the start of his side's innings - when his side is batting second. The Laws stipulate a maximum time for rolling the pitch - no longer than seven minutes.
If there is more than one roller available the batting captain must be given the choice of which one he wants used.
The rolling must not occur earlier than 30 minutes before play begins and must have started not less than 10 minutes before the scheduled or re-scheduled time for play.
Apart from the rolling carried out by the groundstaff before the match, any subsequent rolling does not have to be for the full seven minutes - the batting captain can choose how long or, indeed, whether he wants the pitch rolled at all. What he cannot choose is which bit of the pitch is to be rolled. It's all of it or none of it.