American Bar Association Model Rules of Professional Conduct - Rule 7.5. Firm Names and Letterhead
(a) A lawyer shall not use a firm name, letterhead or other professional designation that violates Rule 7.1. A trade name may be used by a lawyer in private practice if it does not imply a connection with a government agency or with a public or charitable legal services organization and is not otherwise in violation of Rule 7.1.
(b) A law firm with offices in more than one jurisdiction may use the same name or other professional designation in each jurisdiction, but identification of the lawyers in an office of the firm shall indicate the jurisdictional limitations on those not licensed to practice in the jurisdiction where the office is located.
(c) The name of a lawyer holding a public office shall not be used in the name of a law firm, or in communications on its behalf, during any substantial period in which the lawyer is not actively and regularly practicing with the firm.
(d) Lawyers may state or imply that they practice in a partnership or other organization only when that is the fact.