The first thing most lawyers learn about conflicts of interest is the substantial-relationship rule. With few exceptions, a lawyer may not represent a client in a matter adverse to a former client if the present and former matters are substantially related. The substantial-relationship rule, now incorporated in Texas Disciplinary Rule of Professional Conduct 1.09(a)(3), has long served as the touchstone for deciding whether a lawyer may take on a matter against a former client.
The substantial-relationship rule is premised on an irrebuttable presumption that the former client shared confidences with the lawyer that could be useful to the new client. This conclusive presumption allows the former client to move to disqualify the lawyer without disclosing confidences or even proving that it actually shared any confidences. The presumption also means that the lawyer may not defend against disqualification by denying the receipt of confidences from the client. So long as the new matter is adverse to the former client and the new matter is substantially related to the old one, disqualification is automatic and mandatory.
