concurring.
I join Judge Prost’s opinion but with the following observations.
1. In my view, the question whether the PTO is authorized to promulgate particular regulations does not turn on an abstract inquiry into whether a particular rule can be characterized as substantive, procedural, or interpretive. Instead, it calls on us to ask what Congress has empowered the PTO to do through rulemak-ing. Congress has not used the broadest available language in the statute that authorizes the PTO to engage in rulemaking, but neither has it used the narrowest. Congress could have authorized the PTO to issue any regulations that are necessary or appropriate to administer the patent laws. See, e.g., 38 U.S.C. § 501 (Secretary of Veterans Affairs authorized to prescribe “all rules and regulations which are neces-, sary or appropriate to carry out the laws administered by the Department”); 5 U.S.C. § 8347(a) (Office of Personnel Management given authority to prescribe “such regulations as are necessary and proper to carry out [the Civil Service Retirement Act]”). Language of that sort would have given the PTO the very broad rulemaking authority. On the other hand, the PTO could have given no special authority to promulgate regulations, which would have had the effect of limiting the PTO to the narrow scope of 5 U.S.C. § 301, which allows all agencies to prescribe regulations “for the government of ... [the] department, the conduct of its employees, the distribution and performance of its business, and the custody, use, and preservation of its records, papers, and property.” Congress did neither. Instead, it charted a middle course in 35 U.S.C. § 2(b), permitting the agency somewhat broader regulatory powers than are contemplated by section 301, but narrower than the broad “necessary or appropriate” rulemaking authority given to some other agencies.
Section 2(b)(2)(A) of the Patent Act vests the PTO with authority to promulgate regulations that “govern the conduct of proceedings in the Office.” The subject matter that most clearly falls within the scope of that provision is the admission and discipline of attorneys practicing before the PTO. See, e.g., Bender v. Dudas, 490 F.3d 1361, 1368 (Fed.Cir.2007); Lacavera v. Dudas, 441 F.3d 1380, 1383 (Fed.Cir.2006). Even apart from that context, however, we have taken a fairly expansive view of the scope of section 2(b)(2)(A). For example, in In re Sullivan, we held that section 2(b)(2)(A) authorized the PTO to promulgate a regulation permitting conferences between an administrative patent judge and the parties to an interference proceeding. 362 F.3d 1324, 1328 (Fed.Cir.2004). Also in the interference context, we held in Stevens v. Tamai that the PTO acted within its authority under section *13662(b)(2)(A) when it promulgated regulations establishing that the movant has the burden of proof and duty of translating earlier filed documents into English, so as to show that the international application contains the same disclosure as the national stage application. 366 F.3d 1325, 1332 (Fed.Cir.2004). Finally, in Cooper Technologies Co. v. Dudas, we held that the PTO was entitled to promulgate a regulation defining the term “original application” in a statutory provision that established the procedures for inter partes reexamination, and that the regulation was entitled to Chevron deference. 536 F.3d 1330, 1336-38 (Fed.Cir.2008).
For essentially the reasons given by the majority opinion and in light of the above-cited authorities, I am satisfied that the regulations in this case are of the type that Congress authorized in section 2(b) of the Patent Act, as that provision has been construed by this court. While I think it is generally fair to characterize that statute as authorizing the promulgation of “procedural” regulations, however, I do not think it necessary, or particularly helpful, to consider whether those regulations would be deemed “substantive,” “interpretive,” or “procedural” either under section 4 of the Administrative Procedure Act, 5 U.S.C. § 553, or under statutory schemes applicable to other agencies.
The same approach seems to me to be called for in deciding whether the agency is entitled to deference with respect to the scope of its own authority. There, too, the issue comes down to one of statutory construction — whether Congress left the boundaries of agency authority undefined and subject to refinement through the exercise of agency expertise, or whether Congress established a firm line, enforceable by courts, beyond which the agency could not venture. Normally, Congress defines the field of agency rulemaking authority in unambiguous terms that are readily applied by courts, so there is little reason to resort to Chevron-type analysis. In some instances, however, Congress has defined the agency’s rulemaking jurisdiction using vague terms, or terms that call for agency interpretation, or even terms that expressly leave the scope of rulemak-ing authority to the agency to decide; in those cases, deference to the agency may be appropriate or even necessary. See, e.g., Bender v. Dudas, 490 F.3d 1361, 1368 (Fed.Cir.2007) (deference given to PTO’s interpretation of the phrase “before the Office” in section 2(b)(2)); Enercon GmbH v. Int’l Trade Comm’n, 151 F.3d 1376, 1380-81 (Fed.Cir.1998) (deference given to ITC’s interpretation of the word “sale” in the statute giving it jurisdiction, 15 U.S.C. § 1337); 5 U.S.C. § 7701(a) (authorizing Merit Systems Protection Board to act on appeals authorized by “any law, rule, or regulation”). In this case, it is unnecessary to decide whether deference would be due to the agency’s interpretation of its own authority, as we conclude, even without deference, that the agency has authority to issue regulations of the sort issued in this case, subject to their consistency with the underlying statutory provisions being interpreted.
Because I agree with Judge Prost that the challenged regulations are within the scope of the authorization that Congress granted to the PTO in section 2(b), I likewise conclude that the issue in this case comes down to whether the challenged regulations are consistent with other provisions of the Patent Act.
2. On the merits, the most difficult question in this case for me is whether Final Rule 78 is a valid regulation in light of 35 U.S.C. § 120. My colleagues conclude that it is invalid, although for different reasons. I agree that it is invalid for the reasons given by Judge Prost, al*1367though I think it is important to emphasize the narrow scope of the court’s decision.
The court holds that Final Rule 78 is invalid because it limits the number of continuation applications that may be filed and applies that limit even if all of the continuation applications are filed while the first application is still pending. Section 120 plainly provides that any application that satisfies the other requirements of the statute and is “filed before the patenting or abandonment of or termination of proceedings on the first application” shall have the same effect “as though filed on the date of the prior application.” 35 U.S.C. § 120. Therefore, a rule limiting the number of continuances co-pending with the first-filed application is necessarily contrary to the statute and invalid.
While that is a sufficient reason to invalidate Final Rule 78, it does not answer the question whether the rule is invalid as applied to serial continuances, i.e., a series of continuances in which each was co-pending with its immediate predecessor, but in which only the second in the series was co-pending with the first application. Under current law, all continuances in such a series, if they satisfy the other requirements of section 120, are deemed to have the same effective date as the first application. Rule 78 would change that practice.
The question whether the new Rule’s restrictions on serial continuances would also be invalid is more complex than the question of the validity of restrictions on co-pending applications. As to serial continuances, section 120 provides that an application for continued prosecution is entitled to the benefit of an earlier priority date when it is co-pending with “an application similarly entitled to the benefit of the filing date of the first application.” For the last 40 years, that portion of section 120 has been understood to confer upon patent applicants the right to file any number of successive continuation applications after the first application has been abandoned or issued as a patent. That was the construction of section 120 that our predecessor court adopted in 1968, overturning a Patent Office Board of Appeals decision to the contrary. In re Henriksen, 55 C.C.P.A. 1384, 399 F.2d 253, 254 (1968). It would not be unreasonable, however, to construe the phrase “an application similarly entitled” to mean an application that satisfies all the preceding requirements set forth in section 120, including the requirement of co-pendency with the initial application, which was the construction adopted by the Patent Office Board of Appeals in the Henriksen case. See Ex parte Henriksen, 154 U.S.P.Q. 53 (1966). In fact, the court in Henriksen acknowledged that a literal reading of the statutory language would lead to that conclusion. In re Henriksen, 399 F.2d at 256, 260-61 & n. 18. Under that interpretation, applicants would be limited to a maximum of two continuations in series — one while the first application is pending and another while the first continuation is pending. Because the term “similarly entitled” admits of two reasonable constructions, the PTO could have adopted the narrower construction notwithstanding prior judicial precedent construing the statute in the absence of a regulation interpreting the statutory language. See Nat’l Cable & Telecomms. Assn’ v. Brand X Internet Servs., 545 U.S. 967, 982-86, 125 S.Ct. 2688, 162 L.Ed.2d 820 (2005).
The court today properly strikes down Final Rule 78 because its restrictions on co-pending, or parallel, continuations is contrary to the plain language of section 120, which provides that such a co-pending continuation “shall” be given the same priority date as the original application and which contains no restriction on the numbers of such applications that are permitted. That is not to say, however, that a revised rule that addressed only serial con*1368tinuances and limited such continuances to only two — the first co-pending with the original application and the second co-pending with the first — would be struck down as reflecting an impermissible interpretation of section 120. That is not a question that we need to — or should — decide today, but in my view it is important to emphasize that the question remains open.