The American Bar Association's Model Rules of Professional Conduct are designed to guide lawyers and create a structure for regulating attorney conduct. The Model Rules have been adopted (with various modifications) by most states and the District of Columbia. The ABA has amended the Rules and associated Comments several times since their promulgation in 1983; always check the currency of whatever source you choose.
The ABA's Standing Committee on Ethics and Professional Responsibility issues ethics opinions that construe the Model Rules of Professional Conduct. (Prior to 1983, the Committee construed the Model Code of Professional Responsibility; earlier forms of the committee interpreted the Canons of Professional Ethics.) These ethics opinions are not binding on any court or disciplinary authority, but are considered highly persuasive.
Formal Opinions are interpretations of the rules that the Committee believes to be of general interest. Informal Opinions address questions that are narrower in scope and have arisen less frequently. The Committee has not published any informal opinions since 1989.