information:filtering
Adversarial system vs. inquisitorial system

Common law systems tend to give more weight to separation of powers between the judicial branch and the executive branch. In contrast, civil law systems are typically more tolerant of allowing individual officials to exercise both powers. One example of this contrast is the difference between the two systems in allocation of responsibility between prosecutor and adjudicator.

Common law courts usually use an adversarial system, in which two sides present their cases to a neutral judge. For example, in criminal cases, in adversarial systems, the prosecutor and adjudicator are two separate people. The prosecutor is lodged in the executive branch, and conducts the investigation to locate evidence. That prosecutor presents the evidence to a neutral adjudicator, who makes a decision.

In contrast, in civil law systems, criminal proceedings proceed under an inquisitorial system in which an examining magistrate serves two roles by first developing the evidence and arguments for one side and then the other during the investigation phase. The examining magistrate then presents the dossier detailing his or her findings to the president of the bench that will adjudicate on the case where it has been decided that a trial shall be conducted. Therefore, the president of the bench's view of the case is not neutral and may be biased while conducting the trial after the reading of the dossier. Unlike the common law proceedings, the president of the bench in the inquisitorial system is not merely an umpire and is entitled to directly interview the witnesses or express comments during the trial, as long as he or she does not express his or her view on the guilt of the accused.

The proceeding in the inquisitorial system is essentially by writing. Most of the witnesses would have given evidence in the investigation phase and such evidence will be contained in the dossier under the form of police reports. In the same way, the accused would have already put his or her case at the investigation phase but he or she will be free to change his or her evidence at trial. Whether the accused pleads guilty or not, a trial will be conducted. Unlike the adversarial system, the conviction and sentence to be served (if any) will be released by the trial jury together with the president of the trial bench, following their common deliberation.

In contrast, in an adversarial system, on issues of fact, the onus of framing the case rests on the parties, and judges generally decide the case presented to them, rather than acting as active investigators, or actively reframing the issues presented. "In our adversary system, in both civil and criminal cases, in the first instance and on appeal, we follow the principle of party presentation. That is, we rely on the parties to frame the issues for decision and assign to courts the role of neutral arbiter of matters the parties present." This principle applies with force in all issues in criminal matters, and to factual issues: courts seldom engage in fact gathering on their own initiative, but decide facts on the evidence presented (even here, there are exceptions, for "legislative facts" as opposed to "adjudicative facts").

On the other hand, on issues of law, common law courts regularly raise new issues (such as matters of jurisdiction or standing), perform independent research, and reformulate the legal grounds on which to analyze the facts presented to them. The United States Supreme Court regularly decides based on issues raised only in amicus briefs from non-parties. One of the most notable such cases was Erie Railroad v. Tompkins, a 1938 case in which neither party questioned the ruling from the 1842 case Swift v. Tyson that served as the foundation for their arguments, but which led the Supreme Court to overturn Swift during their deliberations. To avoid lack of notice, courts may invite briefing on an issue to ensure adequate notice. However, there are limits—an appeals court may not introduce a theory that contradicts the party's own contentions.

There are many exceptions in both directions. For example, most proceedings before U.S. federal and state agencies are inquisitorial in nature, at least the initial stages (e.g., a patent examiner, a social security hearing officer, and so on), even though the law to be applied is developed through common law processes.