Professional Conduct Rules Table of Contents
(a) 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 gives informed consent, confirmed in writing.
(b) A lawyer shall not knowingly represent a person in the same or a substantially related matter in which a firm with which the lawyer formerly was associated had previously represented a client:
(1) whose interests are materially adverse to that person; and
(2) about whom the lawyer had acquired information protected by Rules 1.6 and 1.9(c) that is material to the matter; unless the former client gives informed consent, confirmed in writing.
(c) A lawyer who has formerly represented a client in a matter or whose present or former firm has formerly represented a client in a matter shall not thereafter:
(1) use information relating to the representation to the disadvantage of the former client except as these Rules would permit or require with respect to a client, or when the information has become generally known; or
(2) reveal information relating to the representation except as these Rules would permit or require with respect to a client.
Ethics Committee Comment
The New Hampshire Supreme Court has relied upon this rule for the criteria governing the consideration of a motion to disqualify a party's former lawyer for a conflict of interest. Sullivan County Reg. Refuse Disp. Dist. v. Town of Acworth, 141 N.H. 479, 481-82 (1996).
Law firms and legal service organizations which handle a high volume of cases confront the limitations of this rule on a more frequent basis than do other practitioners. Firms and organizations may accept cases where a former client is a witness in the new (current client's) case if the representation of the former client is not “substantially related” to the current client’s case. Rule 1.9(a) permits such representation, but attorneys are cautioned to fully explore the definition of "substantially related" under relevant case law in the controlling jurisdiction. If such representation is permissible, attorneys in the law firm or organization must nevertheless take appropriate steps in a case that is not substantially related to comply with Rule 1.9(c) by protecting the confidential information obtained during the representation of the former client.
The New Hampshire Public Defender has adopted a Rule 1.9(c) compliance policy in cases that are not substantially related in which a “neutral attorney” orders the former client’s files sealed and prohibits any communication between the attorney who represented the former client and the attorney who represents the new client. In two cases where the State sought disqualification of the Public Defender because one of its attorneys had previously represented an individual who was a state's witness in the new case, the New Hampshire Superior Court denied disqualification and referenced with apparent approval the Public Defender's Rule 1.9(c) compliance policy. See State of New Hampshire v. Gordon Perry, Nos. 97-S-777 - 780 (Merrimack County Superior Court (Nadeau, J.) April 10, 1998); State of New Hampshire v. Eric Smalley, No. 01-S-1280 (Merrimack County Superior Court (McGuire, J.) January 29, 2002).
2004 ABA Model Rule Comment
RULE 1.9 DUTIES TO FORMER CLIENTS
 After termination of a client-lawyer relationship, a lawyer has certain continuing duties with respect to confidentiality and conflicts of interest and thus may not represent another client except in conformity with this Rule. Under this Rule, for example, 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. Nor could a lawyer who has represented multiple clients in a matter represent one of the clients against the others in the same or a substantially related matter after a dispute arose among the clients in that matter, unless all affected clients give informed consent. See Comment . Current and former government lawyers must comply with this Rule to the extent required by Rule 1.11.
 The scope of a "matter" for purposes of this Rule depends 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 in that transaction 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 factually 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 jurisdictions. 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.
 Matters are "substantially related" for purposes of this Rule if they involve the same transaction or legal dispute or if there otherwise is a substantial risk that confidential factual information as would normally have been obtained in the prior representation would materially advance the client's position in the subsequent matter. For example, a lawyer who has represented a businessperson and learned extensive private financial information about that person may not then represent that person's spouse in seeking a divorce. Similarly, a lawyer who has previously represented a client in securing environmental permits to build a shopping center would be precluded from representing neighbors seeking to oppose rezoning of the property on the basis of environmental considerations; however, the lawyer would not be precluded, on the grounds of substantial relationship, from defending a tenant of the completed shopping center in resisting eviction for nonpayment of rent. Information that has been disclosed to the public or to other parties adverse to the former client ordinarily will not be disqualifying. Information acquired in a prior representation may have been rendered obsolete by the passage of time, a circumstance that may be relevant in determining whether two representations are substantially related. In the case of an organizational client, general knowledge of the client’s policies and practices ordinarily will not preclude a subsequent representation; on the other hand, knowledge of specific facts gained in a prior representation that are relevant to the matter in question ordinarily will preclude such a representation. A former client is not required to reveal the confidential information learned by the lawyer in order to establish a substantial risk that the lawyer has confidential information to use in the subsequent matter. A conclusion about the possession of such information may be based on the nature of the services the lawyer provided the former client and information that would in ordinary practice be learned by a lawyer providing such services.
Lawyers Moving Between Firms
 When lawyers have been associated within a firm but then end their association, the question of whether a lawyer should undertake representation is more complicated. There are several competing considerations. First, the client previously represented by the former firm must be reasonably assured that the principle of loyalty to the client is not compromised. Second, the rule should not be so broadly cast as to preclude other persons from having reasonable choice of legal counsel. Third, the rule should not unreasonably hamper lawyers from forming new associations and taking on new clients after having left a previous association. In this connection, it should be recognized that today many lawyers practice in firms, that many lawyers to some degree limit their practice to one field or another, and that many move from one association to another several times in their careers. If the concept of imputation were applied with unqualified rigor, the result would be radical curtailment of the opportunity of lawyers to move from one practice setting to another and of the opportunity of clients to change counsel.
 Paragraph (b) operates to disqualify the lawyer only when the lawyer involved has actual knowledge of information protected by Rules 1.6 and 1.9(c). Thus, if a lawyer while with one firm acquired no knowledge or information relating to a particular client of the firm, and that lawyer later joined another firm, neither the lawyer individually nor the second firm is disqualified from representing another client in the same or a related matter even though the interests of the two clients conflict. See Rule 1.10(b) for the restrictions on a firm once a lawyer has terminated association with the firm.
 Application of paragraph (b) depends on a situation's particular facts, aided by inferences, deductions or working presumptions that reasonably may be made about the way in which lawyers work together. A lawyer may have general access to files of all clients of a law firm and may regularly participate in discussions of their affairs; it should be inferred that such a lawyer in fact is privy to all information about all the firm's clients. In contrast, another lawyer may have access to the files of only a limited number of clients and participate in discussions of the affairs of no other clients; in the absence of information to the contrary, it should be inferred that such a lawyer in fact is privy to information about the clients actually served but not those of other clients. In such an inquiry, the burden of proof should rest upon the firm whose disqualification is sought.
 Independent of the question of disqualification of a firm, a lawyer changing professional association has a continuing duty to preserve confidentiality of information about a client formerly represented. See Rules 1.6 and 1.9(c).
 Paragraph (c) provides that information acquired by the lawyer in the course of representing a client may not subsequently be used or revealed by the lawyer to the disadvantage of the client. However, the fact that a lawyer has once served a client does not preclude the lawyer from using generally known information about that client when later representing another client.
 The provisions of this Rule are for the protection of former clients and can be waived if the client gives informed consent, which consent must be confirmed in writing under paragraphs (a) and (b). See Rule 1.0(e). With regard to the effectiveness of an advance waiver, see Comment  to Rule 1.7. With regard to disqualification of a firm with which a lawyer is or was formerly associated, see Rule 1.10.
Professional Conduct Rules Table of Contents