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[1][2] Federal Rule of Evidence 501 provides that "the privilege of a witness .. shall be governed by the principles of the common law as they may be interpreted by the courts of the United States in light of reason and experience." The attorney-client privilege is the oldest of the privileges for confidential communications known to the common law. 8 J. Wigmore, Evidence (section) 2290 (McNaughton rev. 1961). Its purpose is to encourage full and frank communication between attorneys and their clients and thereby promote broader public interests in the observance of law and administration of justice. The privilege recognizes that sound legal advice or advocacy serves public ends and that such advice or advocacy depends upon the lawyer's being fully informed by the client. As we stated last Term in Trammel v. United States, 445 U.S. 40, 51, 100 S.Ct. 906, 913, 63 L.Ed.2d 186 (1980): "The lawyer-client privilege rests on the need for the advocate and counselor to know all that relates to the client's reasons for seeking representation if the professional mission is to be carried out." And in Fisher v. United States, 425 U.S. 391, 403, 96 S.Ct. 1569, 1577, 48 L.Ed.2d 39 (1976), we recognized the purpose of the privilege to be "to encourage clients to make full disclosure to their attorneys." This rationale for the privilege has long been recognized by the Court, see Hunt v. Blackburn, 128 U.S. 464, 470, 9 S.Ct. 125, 127, 32 L.Ed. 488 (1888) (privilege "is founded upon the necessity, in the interest and administration of justice, of the aid of persons having knowledge of the law and skilled in its practice, which assistance can only be safely and readily availed of when free from the consequences or the apprehension of disclosure"). Admittedly complications in the application of the privilege arise when the client is a corporation, which in theory is an artificial creature of the 390law, and not an individual; but this Court has assumed that the privilege applies when the client is a corporation. United States v. Louisville & Nashville R. Co., 236 U.S. 318, 336, 35 S.Ct. 363, 369, 59 L.Ed. 598 (1915), and the Government does not contest the general proposition.

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The narrow scope given the attorney-client privilege by the court below not only makes it difficult for corporate attorneys to formulate sound advice when their client is faced with a specific legal problem but also threatens to limit the valuable efforts of corporate counsel to ensure their client's compliance with the law. In light of the vast and complicated array of regulatory legislation confronting the modern corporation, corporations, unlike most individuals, "constantly go to lawyers to find out how to obey the law," Burnham, The Attorney-Client Privilege in the Corporate Arena, 24 Bus.Law. 901, 913 (1969), particularly since compliance with the law in this area is hardly an instinctive matter, see, e. g., United States v. United States Gypsum Co., 438 U.S. 422, 440-441, 98 S.Ct. 2864, 2875-2876, 57 L.Ed.2d 854 (1978) ("the behavior proscribed by the [Sherman] Act is 393often difficult to distinguish from the gray zone of socially acceptable and economically justifiable business conduct").1 The test adopted by the court below is difficult to apply in practice, though no abstractly formulated and unvarying "test" will necessarily enable courts to decide questions such as this with mathematical precision. But if the purpose of the attorney-client privilege is to be served, the attorney and client must be able to predict with some degree of certainty whether particular discussions will be protected. An uncertain privilege, or one which purports to be certain but results in widely varying applications by the courts, is little better than no privilege at all. The very terms of the test adopted by the court below suggest the unpredictability of its application. The test restricts the availability of the privilege to those officers who play a "substantial role" in deciding and directing a corporation's legal response. Disparate decisions in cases applying this test illustrate its unpredictability. Compare, e. g., Hogan v. Zletz, 43 F.R.D. 308, 315-316 (ND Okl.1967), aff'd in part sub nom. Natta v. Hogan, 392 F.2d 686 (CA10 1968) (control group includes managers and assistant managers of patent division and research and development department), with Congoleum Industries, Inc. v. GAF Corp., 49 F.R.D. 82, 83-85 (ED Pa.1969), aff'd, 478 F.2d 1398 (CA3 1973) (control group includes only division and corporate vice presidents, and not two directors of research and vice president for production and research).