Model Rule 1.9 governs conflicts with former clients. What is the core principle?

• A. Do not reveal confidential information or represent in a matter substantially related where the interests are adverse, unless informed consent is obtained.
• B. Former clients have no protections.
• C. A lawyer may represent a former client in any matter after a short waiting period.
• D. The former client is automatically waived if the matter is unrelated.

Answer: (A)

The main idea is that a lawyer who formerly represented a client must protect that client’s interests even after the representation ends. Model Rule 1.9 bars a lawyer from taking on a new matter that is the same or substantially related to the former matter if the former client’s interests are adverse, unless the former client provides informed consent in writing. It also forbids using or revealing confidential information acquired during the prior representation to the detriment of the former client.

This means two things at once: you cannot represent someone else in a case that is substantially related to what you handled for the former client when the new client's interests oppose the former client's interests, unless you have written consent from the former client. And you cannot disclose or use confidential information you learned from the former client in any way that could harm them.

That’s why the stated principle is the best answer: it embodies both the duty not to reveal confidential information and the duty not to represent against a former client in a substantially related matter without informed written consent. The other options misstate protections, introduce an automatic waiting period, or imply waivers exist for unrelated matters, which they do not.