By comity between foreign governments judgments of courts in one country are usually treated as valid in another country; that is to say, if in an action brought in the courts of one country having jurisdiction of the case, a judgment is rendered determining the rights of the parties to the action, this judgment is regarded as conclusive between the same parties in an action involving the same issues brought in another country, and can only be impeached or disregarded on proof that the judgment was not valid where rendered. This rule of comity existing between countries wholly foreign to each other is made a constitutional rule as between the states by provision of the federal constitution that " Full faith and credit shall be given in each state to the public acts, records, and judicial proceedings of every other state. And the Congress may by general laws prescribe the manner in which such acts, records, and proceedings shall be proved and the effect thereof" (Const. Art. IV, § 1, ¶ 1). This provision extends not only to judgments rendered and the records thereof, but also to the public statutes of a state, so that if it becomes necessary to determine in the courts of one state what the public statutes of another state are, the fact may be shown by the legislative records of the state whose laws are in question.

Congress has made provision (first in 1790) for the method of proving in any state the public acts, records, and judicial proceedings of any other state. The public laws of any state are presumed to be known to persons within that state and the courts of the state will take judicial notice of them without proof; but the laws of another state are not thus presumed to be matter of general knowledge and the courts will not take judicial notice of them, but they must be proven like other facts when they are in any way called in question. However, the courts of a state must take judicial notice of the constitution, laws, and treaties of the United States; and likewise the courts of the United States held in any state must take judicial notice of the constitution and laws of that state. The laws of another state or country are to be proved as matters of fact; a court cannot in general take judicial notice of them.