In cases arising under the constitution, laws, or treaties of the United States, and depending for their decision on the construction thereof, the federal courts follow their own judgment, guided, of course, by the decisions of the Supreme Court of the United States. But many cases, particularly cases which are in the federal courts by reason of diversity of citizenship of the parties, do not involve the constitution, laws, or treaties of the United States, or any rights arising under them, but only the application of general principles of law, or the construction of the constitutions or statutes of the different states; and in these cases the federal courts apply the general principles of law, or the constitutional or statutory provisions which would have been applied had such cases been tried in the state courts. The federal courts follow the decisions of the state courts in the construction of their own constitutions and statutes, and also in cases involving some established rule of property. But in other cases, which are to be decided according to the general principles of law, the federal courts determine for themselves what such general principles are, without feeling themselves bound to follow the decisions of the particular state (Railroad Company v. National Bank and Burgess v. Seligmari).

In all the states except Louisiana the common-law system, that is, the English system of law, is recognized as in force, so far as consistent with the institutions and conditions under which we live; while in that state the prevailing system of law is that known as the civil law, as embodied in the Code Napoleon, which was in force in France at the time the Louisiana Territory was acquired by the United States. Therefore, in cases tried in the federal courts sitting in any of the states except Louisiana, it is presumed that the rights of parties are determined by the common law, save as it may have been modified or superseded by state statutes; that is, the common law is the general law for each of these states (Smith v. Alabama). It can hardly be said, however, that there is a common law of the United States, for so far as cases are to be determined by general law, in the absence of statutory provisions, the federal courts are presumed to follow the general law as it exists in the state where the federal court sits, or in the state according to the law of which the case is to be decided, although, as already indicated, the federal courts are not bound by the decisions of the state courts interpreting the general law, except so far as the decisions of the state courts have become rules of property. A subject-matter which is placed within the control of Congress, such as interstate commerce, is assumed to be subject to the general rules of the common law so far as Congress has not legislated with reference to it. (See Western Union Telegraph Co. v. Call Publishing Co.)

There are no common-law crimes against the United States, that is to say, no acts are punishable in the federal courts as crimes save as they have been made criminal by federal statutes. However, when a punishment is provided by federal statute for an act generally described by the terms used in the common-law definitions of crimes, the courts will consider the common-law definition as indicating the scope of the language used in the statute.