The petition
The process opens with a written libellus setting out the facts, pleading the ground of nullity — the caput nullitatis — and asking the tribunal to declare it. It is the act that fixes the boundaries of the whole cause. The presiding judge may reject it in the cases listed in can. 1506; recourse against rejection lies within ten days of notification of the decree.
Which tribunal
Can. 1672 gives the place of the wedding, the place of domicile or quasi-domicile of either party, and the place where most of the evidence is to be gathered.
The joinder of the issue
Once the petition is admitted, the tribunal settles the formula of the doubt: the precise question the sentence will have to answer. From that point the investigation proceeds within those limits.
The investigation
This comprises the examination of the parties, the hearing of witnesses, the gathering of documents and, in causes of incapacity to consent, the expert report. The Defender of the Bond intervenes in every cause in support of the validity of the marriage: he is not an opposing party but an officer of the tribunal. The Promotor of Justice intervenes where the regularity of the process is in question.
When the investigation closes, the acts are published: counsel receive a copy, while the parties themselves may read them but not take one away.
Decision and appeal
A first-instance affirmative sentence, if not challenged within fifteen days of notification, becomes executive. If it is challenged, the appeal tribunal — which may be the Roman Rota — either dismisses the appeal as manifestly unfounded, so that the sentence stands, or admits the cause to a full hearing.
Before the Roman Rota
Some features are particular to the Rota: the Defender of the Bond opens the instance with his votum; the formula of the doubt is reduced to whether the nullity of the marriage is established in the case; and the evidence is not gathered afresh in full, but directed to the points genuinely in issue.
The shorter process before the Bishop
The 2015 reform introduced a shorter route, subject to two conditions taken together (can. 1683): that the petition is brought by both spouses or with the consent of both, and that the circumstances make the nullity manifest. The hearing is concentrated in a single session. Outside those cases the ordinary process applies.
The documentary process
Where a document open to no contradiction establishes a diriment impediment or a defect of canonical form, and it is equally certain that no dispensation was granted, nullity may be declared in the form provided by can. 1688.