How were Roman trials divided between a magistrate and a judex?

In the early stages of Roman legal procedure, such as the legis actiones and the formulary system, trials were bifurcated into two distinct phases.1

The Two-Part Trial Structure

• Before the Magistrate: The first phase was a preliminary hearing held before a magistrate. The magistrate’s role was to determine if a valid legal issue existed to be contested. During this stage, the parties delineated the issues of the case and set sureties.1
• Before the Judex: Once the issues were defined, the parties agreed upon a judex—a prominent layman rather than a professional judge or lawyer—to hear the case. Proceedings before the judex were more informal, involving the presentation of evidence, witness testimony, and arguments from advocates.1

While the judex rendered a decision, they lacked the authority to enforce it. If a defendant failed to comply with the judgment, the plaintiff had to return to the magistrate to initiate enforcement, which could involve the seizure of property or personal servitude. This bifurcated system eventually gave way to the cognitio extraordinaria, where the magistrate assumed full responsibility for both the trial and the execution of the sentence.1