Ambitious plans rely on inmate numbers
The feasibility of the government's plans to replace out-of-date prisons with new buildings depends largely on the size of the prison population, which is notoriously hard to predict.
Prisons can't be closed unless there is appropriate prison capacity elsewhere or unless the overall jail population is falling - and there is no sign that it is.
It is currently 85,884 - but it is not rising as quickly as predicted a year ago, when the last official population projections were published.
It was estimated then that by June this year the population would be between 87,100 and 88,900. In fact it was around 86,000, which may have given ministers more confidence that their plans can be achieved.
The projections said that by 2020 the population could be anything between 81,400 and 98,900, with the "central scenario" - the most likely - suggesting it would be 90,200.
|Prison population (England and Wales)|
|Source: The Ministry of Justice|
If the population did edge towards the upper estimates it would be extremely hard for the government's plans to work. They would need to build thousands of extra places, as well as replacing old ones.
New prison population forecasts are due to be published this month.
As for the closures, it's long been speculated that Pentonville, in north London, might be closed. It was heavily criticised by inspectors earlier this year.
Other possible candidates for closure in London include Wormwood Scrubs, Brixton and Wandsworth. Leeds, Manchester and Liverpool are all old prisons, which are known to be very expensive to run.
Dartmoor Prison has already been earmarked for closure. As well as Reading - which the Treasury has said will be first to be sold - several jails are currently unused and could also be sold off.
They include Dover, which was until recently an immigration removal centre, and Downview Prison, in Surrey, which was a women's prison.
But building prisons takes many years.
Plans for a new prison in north Wales were first made public in January 2013, but the proposals were under consideration before that. The new jail, in Wrexham, is not due to open until 2017.
Sites have to be identified and planning permission has to be granted before construction work can even start.
These are ambitious plans.