Doncaster Crown Court will today hear the appeal of Paul Chambers against his conviction under Section 127 of the Communications Act 2003 for sending a menacing message over a public telecommunications system.

The background to this unfortunate case -- and why it matters so much to the public -- is set out both here and here. Among those outraged by this misconceived and illiberal prosecution are commentators as various as Nick Cohen and Graham Linehan.

There is no need to add to this scorn and derision for the Crown Prosecution Service. This blog post serves a different purpose: it offers a very brief guide for those following the case today.

The appeal is a full rehearing of the case. Although Paul is the appellant, the prosecution will still go first.

The prosecution then has to establish three things. First, whether Paul's tweet was a message sent over a public telecommuniations system. Second, that the tweet was itself menacing. And third, that Paul sent it with the intention of sending a menacing communication.

On the last point, it would be enough for the prosecution to show that he was aware of the effect it could have, rather than that it was his only deliberate purpose.

The requirements to show that the tweet was menacing and that Paul had the intention (or awareness) that he was sending a menacing communication together have to be proved by the prosecution to the "criminal standard" -- that is, beyond reasonable doubt.

Once the prosecution has set out its case, it falls to Paul's side to make his defence. Acting for Paul is the renowned criminal defence barrister Stephen Ferguson.

It is expected that Paul will be called to give evidence. It may also be that others may be required to give oral evidence.

But there is little dispute over fact, and so no great need for cross-examination; indeed, most of the facts are agreed. The real question for the court is whether the facts actually add up to an offence.

Hearing the case will be a professional judge accompanied by two justices of the peace. As this is a full appeal, the sentence for Paul could theoretically be increased. The maximum sentence for a Section 127 offence is six months' imprisonment.

That said, it would appear unlikely that the sentence will be varied. Rather, the key issue appears to be whether Paul should have been convicted or not.

The verdict should be given later today. It is rare for a criminal court to reserve judgment on such matters, though possible. If the appeal is allowed, then Paul can leave the court with vindication and without a criminal record. If he loses, however, it is open for him to appeal on points of law to the high court, and then to the Court of Appeal.

Beyond that would perhaps be the Supreme Court (the former House of Lords) and the European Court of Human Rights. However, if the result today is bad, then it may be that Paul would just want to draw a line under the whole matter. No one would blame him if he did.

By the end of this afternoon we should know whether the emphatic and lively campaign to reverse the conviction has succeeded, or whether it has suffered a severe setback. As the person co-ordinating the defence, I am, unsurprisingly, a little apprehensive. But those nerves are as nothing in comparison with the stress that Paul and his partner are under.

Paul is dealing with this whole matter with impressive character. He now has a criminal record, and has lost two jobs, for a tweet that was at worst ill-advised. He is doing incredibly well in the circumstances.

For the CPS to regard that tweet as menacing and intended to be so has implications for any person blogging or emailing. There, but for the grace of a god, go most of us.

Let us see if the case can be closed down today, or whether we have to take it to a higher court. Whatever the outcome, Paul Chambers deseves our warmest regards and support.

David Allen Green is providing pro bono assistance to Paul Chambers. He is head of media at City law firm Preiskel & Co and blogs on legal and policy matters for the New Statesman.