Twelve years is just the starting point

Gary Dobson and David Norris will be sentenced to indefinite detention today. As they were under 18 when they murdered Stephen Lawrence, Mr Justice Treacy must sentence them to be "detained during Her Majesty's pleasure".

However, the judge must also set a minimum term that each defendant must serve before being considered for release on licence. If Dobson and Norris had been 18 in 1993, it would have been 30 years on the basis that the murder was racially motivated. As they were under 18, the starting point for them will be 12 years.

The judge must then take into account any aggravating or mitigating factors. It seems clear that racial motivation is one aggravating factor that the judge may consider. Carrying a knife may be another. The judge has said already that he will consider premeditation.

Even though the starting point in this case is 12 years, detailed consideration of aggravating factors may lead to a minimum sentence of any length.

However, there is one restriction on the judge resulting from the fact that Dobson and Norris committed the murder before the sentencing laws were changed in 2003. Mr Justice Treacy cannot set a minimum term that, in his opinion, is longer than the minimum punishment period that the home secretary would have specified under the old "tariff" arrangements. Working out what that period would be cannot be easy.

But it is important to bear in mind that the minimum period is not a sentence. The sentence is indefinite detention. To compare the minimum period with a fixed sentence, it has to be doubled. So a minimum period of 15 years is the equivalent of a fixed sentence of 30 years, of which the offender would serve no more than half in prison.