The laws of the several states, except where the Constitution or treaties of the United States or Acts of Congress otherwise require or provide, shall be regarded as rules of decision in civil actions in the courts of the United States, in cases where they apply.
28 U.S. Code § 1652 - State laws as rules of decision
Based on title 28, U.S.C., 1940 ed., § 725 (R.S. § 721).
“Civil actions” was substituted for “trials at common law” to clarify the meaning of the Rules of Decision Act in the light of the Federal Rules of Civil Procedure. Such Act has been held to apply to suits in equity.
Changes were made in phraseology.