like, promulgated under 5 U.S.C. § 301 and authority delegated to agency heads. A class called "guidance" includes all rules not promulgated by legislative procedure. Such rules may be published as guidance, guidelines, agency staff manuals, staff instructions, opinion letters, interpretive memoranda, policy statements, guidance manuals for the public, circulars, bulletins, advisories, press releases stating agency position, and the like. The class of "guidance" is almost, but not exactly, coextensive with the union of the sets of interpretative rules, general statements of policy, and housekeeping rules. Agency interpretations: § 553(d) "interpretative rules" vs. Chevron/Auer interpretations Every statute and regulation has some lingering ambiguity. Someone has to have authority to adopt some interpretation, and do so with a minimum of procedural delay. So the law grants every agency the authority to promulgate interpretative rules, and to do so with minimal procedural fuss. By default, most interpretations slot into the *interpretative rule" category of 5 U.S.C. § 553(b). If an interpretation satisfies a long list of criteria, then the interpretation is binding on parties before the agency, courts, and the agency itself, under Chevron U.S.A. Inc. v. Natural Resources Defense Council, Inc. (for agency interpretations of statute) or Auer v. Robbins (for agency interpretations of regulations). (The inquiries under Chevron and Auer are slightly different. But the analytical similarities overshadow the differences. For this short article, we will gloss over the differences, and treat Chevron and Auer together.) Any interpretation that fails any one of the Chevron/Auer eligibility bullets from the list below falls into the residual category of "interpretative rule". Interpretative rules and Skidmore deference Fundamentally, the § 553(b) "interpretative" exemption from notice and comment is a rule of necessity—essentially all laws have some ambiguity, that ambiguity has to be interpreted, and (for public-facing substantive rules) the agency is the party that can do so expeditiously and fairly. Deference follows to the degree the agency demonstrates fairness and diligence in developing its interpretation (under Chevron, Auer, or Skidmore v. Swift & Co., as appropriate). The quid pro quo for an agency's choice to exercise the "interpretative" option, and forego the formalities required for legislative rulemaking or for Chevron or Auer deference, is that the agency has very little enhanced power to enforce its interpretation. If a party challenges the agency's interpretation, an agency's invocation of the "interpretative" exemption surrenders any claim to heightened Chevron or Auer deference, and the interpretation falls