Around 86,000 people are in prisons in the UK.

Adult men in prison are put into groups depending on the crime they were found guilty of and how dangerous they would be to the public if they escaped.

There are four of these groups which are called security categories:

Category A: The most dangerous prisoners who would be a huge threat to the public if they escaped. The prison service aims to make it impossible for these people to escape.

Category B: These prisoners are less of a threat to the public, but still dangerous enough to need levels of security which make it very difficult for them to escape.

Category C: These prisoners are thought to be of very little threat to the public and either don't want to escape or don't have the skills needed to do this.

Category D: These prisoners do not pose a risk to the public and are unlikely to escape. These prisoners can be held in open prisons.