Common law marriage is the first example that comes to mind. While state statutes do not recognize a marital union without proper licensing or a religious ceremony, the common law recognizes the legitimacy of a marriage where the couple cohabitates and presents themselves to the world as being married. Of course, these instances are fact sensitive. About 9 years ago Pennsylvania appellate courts abolished common law marriage within the Commonwealth.

Principles of equity in contract law are another example. There are areas of contract law that are addressed vaguely by statute. Equitable principles under common law are used by courts to avoid unfair results, unjust enrichment or dissolution of contractual obligations where the statutes may be silent.