What does Rule 1.9 prohibit concerning former clients?

Rule 1.9 of the ABA Model Rules of Professional Conduct addresses the obligations of a lawyer concerning former clients. Specifically, it prohibits a lawyer from representing a new client in a matter that is substantially related to a matter in which the lawyer previously represented the former client. This is to ensure the confidentiality of the former client’s information and to avoid any potential conflicts of interest that may arise due to the previous representation.

The reasoning behind this prohibition is rooted in the duty of loyalty and confidentiality that lawyers owe to their clients. A lawyer who has previously represented a client has gained knowledge and insights specific to that client's case, and this information could be used to the disadvantage of the former client if the lawyer represents someone else in a related matter.

This rule is designed to maintain the integrity of the attorney-client relationship, even after the official representation has ended. Thus, the prohibition against representing a new client in a matter related to a former client’s case is essential in upholding ethical standards within legal practice.