Lex fori is literally translated from Latin to “law of the forum.” What that means in practical terms is all of the law that relates to the procedure or formalities in a given legal forum. Typically, lex fori is a concept that becomes relevant in situations where the substantive law covering the matter at hand is different from the substantive law of another valid venue where the matter might be tried.
Procedural law must be differentiated from substantive law, called the lex causae. Take, for example, a situation in which a cause of action is derived from events that involve the law of one jurisdiction, but the case might be tried in a second jurisdiction. If the party who determines venue seeks to try the case in the second jurisdiction, then lex fori of the second jurisdiction is applied, and the lex causae — the substantive law — from the first jurisdiction shall be applied. In simple terms, the legal procedures followed in the case are determined by the laws where the trial is being held, but the outcome of the case is determined by the laws where the action originally took place.
The reason behind this is the problem of what is called “forum shopping.” In the event that the person who brings the case can choose between multiple venues that have different substantive law, there are times when the substantive law in one of the venues benefits one party more than the other in that particular cause of action. The application of the substantive law from the jurisdiction where the cause of action arose and the application of the lex fori of the venue where it is tried solves this problem.
Lex fori might include major issues such as right to a jury trial and burden of proof. Typically, though, it refers to less prominent issues such as the procedure for filing motions, pretrial hearings, the protocol for calling witnesses and the like. These issues might affect how the case goes, but they do not directly affect the outcome in terms of how the relevant law is applied. Thus, the selection of lex fori by one party will not unfairly prejudice the second party the way selection of lex causae might.