Former Rules of Professional Conduct: Rule 1.9--Conflict of Interest: Former Client
This Rule governed the practice of law in the District of Columbia from January 1, 1991, through January 31, 2007. As of February 1, 2007, the Amended Rules took effect.
A lawyer who has formerly represented a client in a matter shall not thereafter represent another person in the same or a substantially related matter in which that person’s interests are materially adverse to the interests of the former client unless the former client consents after consultation.
 After termination of client-lawyer relationship, a lawyer may not represent another client except in conformity with the Rule. The principles in Rule 1.7 determine whether the interests of the present and former client are adverse. Thus, a lawyer could not properly seek to rescind on behalf of a new client a contract drafted on behalf of the former client. So also a lawyer who has prosecuted an accused person could not properly represent the accused in a subsequent civil action against the government concerning the same transaction.
 The scope of a "matter" for purposes of this Rule may depend on the facts of a particular situation or transaction. The lawyer’s involvement in a matter can also be a question of degree. When a lawyer has been directly involved in a specific transaction, subsequent representation of other clients with materially adverse interests clearly is prohibited. On the other hand, a lawyer who recurrently handled a type of problem for a former client is not precluded from later representing another client in a wholly distinct problem of that type even though the subsequent representation involves a position adverse to the prior client. Similar considerations can apply to the reassignment of military lawyers between defense and prosecution functions within the same military jurisdiction. The underlying question is whether the lawyer was so involved in the matter that the subsequent representation can be justly regarded as a changing of sides in the matter in question. Rule 1.9 is intended to incorporate federal case law defining the "substantial relationship" test. See, e.g., T.C. Theatre Corp. v. Warner Brothers Pictures, 113 F. Supp. 265 (S.D.N.Y. 1953), and its progeny; see also Conflicts of Interest in the Legal Profession, 94 Harv. L. Rev. 1244, 1315-34 (1981).
 Disqualification from subsequent representation is for the protection of clients and can be waived by them. A waiver is effective only if there is disclosure of the circumstances, including the lawyer’s intended role in behalf of the new client. The question of whether a lawyer is personally disqualified from representation in any matter on account of successive government and private employment is governed by Rule 1.11 rather than by Rule 1.9.
 With regard to an opposing party’s raising a question of conflict of interest, see Comment to Rule 1.7. With regard to disqualification of a firm with which a lawyer is associated, see Rules 1.10 and 1.11.