This appeal is from the decision of the Patent and Trademark Office Board of Patent Interferences (board) awarding priority of invention to Winchell1 over Tofe.2 On the ground that the matter was not one “ancillary to priority,” the board refused to consider whether the Winchell application contained a disclosure of Winchell’s best mode of practicing his invention as required by 35 U.S.C. § 112.3 We vacate the award of priority to Winchell and remand.
Background
The applications of both parties were filed on April 30, 1975 4 Tofe relied upon stipulated testimony as evidence of conception of the invention on April 23, 1975, the execution date of the application, with the filing date serving as the date of constructive reduction to practice. Winchell submitted testimony and documentary evidence from which the board found prior conception and actual reduction to practice as early as October 23,1974. This finding is not attacked by Tofe directly.
Tofe asserts it became clear upon completion of the Winchell rebuttal testimony that Winchell failed to disclose the best mode contemplated by him for carrying out his invention as required by the first paragraph of 35 U.S.C. § 112.5 Tofe contends that because of Winchell’s failure to comply with 35 U.S.C. § 112, Winchell’s application is incomplete under 35 U.S.C. § 1116 and should be denied a filing date and the status of an application, and that since 35 U.S.C. § 1357 requires “an application” which would interfere with another pending application, Winchell has no standing as an interference party. Tofe concludes, therefore, the award of priority should be in his favor.
Tofe appeals from the decision on priority and from the denial of two motions, both predicated on Winchell’s alleged failure to meet the requirements of 35 U.S.C. § 112.
In a belated motion under 37 CFR 1.231(a), invoking 37 CFR 1.258, Tofe asked the board to deny the Winchell application a filing date on the ground that to merit consideration as a bona fide application under 35 U.S.C. § 111, the specification must satisfy the statutory requirements of 35 U.S.C. § 112.
In the other motion, filed under 37 CFR 1.243, Tofe asked that:
*60[T]he Board of Patent Interferences direct the attention of the Commissioner to the failure of the application of the party Winchell to satisfy the best mode requirement of 35 USC 112, paragraph one, and to recommend under 35 CFR 1.259 that the application of the party Winchell be denied its filing date for failure to comply with the statutory requirements of 35 USC 112 and 35 USC 111.
Winchell urged that the issue of no interference in fact could not be predicated on failure to disclose the best mode and that the issue of best mode was not ancillary to priority in this case. Decisions on the motions were deferred by the interference examiner for consideration at final hearing.
The Board
The board first considered the motion under 37 CFR 1.231(a) which asked the board itself to deny Winchell standing in the interference. In denying this motion, the board stated:
We know of no precedent where the Board of Interferences has denied a filing date of an application in an interference proceeding for any reason and then held that there was no interference in fact, and Tofe has offered no precedent for such action. Moreover, we agree with Winchell that 37 CFR 1.231(a)(1) precludes consideration of a motion based on the ground that there is no interference in fact unless the count differs from the corresponding claim of one of the involved applications. [Footnote omitted.] The count in this interference does not differ from the corresponding claims of the parties’ applications.
The Tofe motion also seeks a determination of whether the Winchell application involved in this interference complies with the “best mode” requirement of 35 USC 112. However, this Board views the decision in Thompson v. Dunn, 35 CCPA 957, 166 F.2d 443, 77 USPQ 49 (1948) as prevailing authority for the position that the issue of “best mode” in an application involved in an interference is not ancillary to priority, and therefore this Board is without jurisdiction to consider the issue. We recognize that the Thompson decision was based on R.S. 4888 which preceded Title 35. However, from a comparison of the two statutes, we find no reason to conclude that a different result should now follow. Tofe argues that under Weil v. Fritz, 572 F.2d 856, 196 USPQ 600 (CCPA 1978) the best mode issue is ancillary to priority, and the board has the authority to decide the best mode issue. In our view, however, the decision in Weil v. Fritz, supra, was limited to situations involving a benefit application where the issue of best mode in a benefit application may be coupled with a motion to shift the burden of proof.
Turning to the alternative motion, under 37 CFR 1.243, the board declined to make a recommendation to the Commissioner under 37 CFR 1.259 that the filing date be denied, stating:
With respect to Tofe’s alternative motion requesting that the Board of Interferences recommend to the Commissioner of Patents and Trademarks pursuant to 37 CFR 1.259 that Winchell be denied the filing date of its application for failure to comply with the best mode requirement of 35 USC 112, we note that the making of recommendations under 37 CFR 1.259 is a matter “wholly within the discretion” of the Board. Rivise and Caesar, Interference Law and Practice, Volume IV, Section 813, page 2839 (Michie Co. 1948). We decline to make the recommendation requested. Cf. Brader v. Schaeffer, 193 USPQ 627 (Bd.Pat.Int.1976). [Footnote omitted.]
The board gave no further consideration to the “best mode” question and on the basis of Winchell’s evidence of prior conception and reduction to practice awarded priority of invention to Winchell.
OPINION
We hold that the issue of whether an inventor has failed to disclose in his application for a patent the best mode contemplated by him for practicing his invention as required by 35 U.S.C. § 112, first paragraph, presents an issue which is ancillary *61to priority and is within the jurisdiction of the board and this court to determine.
The parties here have argued the case on the basis that either Winchell or Tofe must eventually emerge from this proceeding with an award of priority.8 Winchell asks that we affirm the validity of Thompson v. Dunn, 35 CCPA 957, 166 F.2d 443, 77 USPQ 49 (1948), in which it was stated that the requirement of disclosure of best mode is not a matter related to priority. Tofe asks that we specifically overrule Thompson v. Dunn, supra, on the ground that the decision was made under an earlier statute in which the best mode requirement served a different purpose from the requirement of the current statute.9 Tofe further argues that Weil v. Fritz, 572 F.2d 856, 196 USPQ 600 (CCPA 1978), is a controlling precedent, a case which will be considered in turn.
With respect to the holding in Thompson v. Dunn, supra, the issue actually being addressed by the court was whether Thompson was entitled to an award of priority if Dunn had failed to disclose his best mode. The thrust of this court’s decision was that that issue was not determinative of priority. The specific holding continues to be correct.10 In so holding, however, the court stated that the issue of best mode fulfillment was not one which “relates to priority of invention” and could not be considered in an interference.
Subsequently, this court has recognized that the jurisdiction of the board necessarily includes certain preliminary questions which, while not determinative of priority, are logically related to that determination so that a decision on the specific preliminary question may moot the issue of priority, Nitz v. Ehrenreich, 537 F.2d 539, 190 USPQ 413 (CCPA 1976). In Nitz this court stated:
The existence of common subject matter defined by the interference count is a prerequisite for an award of priority, i. e., the existence or nonexistence of interfering subject matter goes to the very foundation on which an interference rests. Determination of the presence or absence of interfering subject matter is “logically related” to the jurisdiction-conferring issue of priority because that determination necessarily precedes a priority award. [537 F.2d at 543.]
*62The validity of the holding in Nitz v. Ehrenreich, supra, is supported by the principle that any party to a justiciable controversy may raise the point that there is an absence of jurisdictional facts at any time. Cf. In re Haas, 486 F.2d 1053, 179 USPQ 623 (CCPA 1973); Menchaca v. Chrysler Credit Corp., 613 F.2d 507 (CA 5 1980); Arrowsmith v. United Press International, 320 F.2d 219 (CA 2 1963) (jurisdictional question should be considered before reaching the merits); Boone v. Kurtz, 617 F.2d 435 (CA 5 1980).
We find the logic compelling that 35 U.S.C. § 135, which provides jurisdiction to the board to settle the matter of priority of invention between two applicants for patents on the same invention, contemplates that two valid applications exist. It is, of course, correct that the determination that there should be an interference is initially a matter of discretion residing solely in the Commissioner of Patents and Trademarks, his decision being based on the ex parte record showing two applications ready for allowance except for a determination of priority. However, the failure of an applicant to disclose his best mode is a jurisdictional fact more likely to be brought out by an adverse party in an inter partes proceeding than by the examiner during ex parte prosecution. When the issue of whether the best mode requirement has been satisfied by a party is raised in an interference, we hold that the issue must be resolved before the issue of priority is addressed. We interpret 37 CFR 1.25811 as a sufficiently broad delegation of authority to the board to make its own conclusion on whether an application satisfies the best mode requirement of 35 U.S.C. § 112, based on its judgment of facts not fully appreciated until after the interference was declared. Where a matter has been adjudged to be of such significance that no meaningful determination of priority can be made until the issue is resolved, we have previously required that appropriate procedures be followed below to secure its resolution prior to appeal. For example, in Peska v. Satomu-ra, 602 F.2d 969, 202 USPQ 726 (CCPA 1979), this court, speaking through Judge Miller, ordered on rehearing:
We adhere to our decision vacating the board’s award of priority to Satomura and remanding the case for further proceedings consistent with the original opinion which, however, is hereby modified as follows: our holding that the board erred in refusing to set the case down for final hearing on the issue of whether Peska et al. are entitled to the benefit of their earlier filed Czech application, is revoked. This, we believe, will permit the PTO in further proceedings to follow such internal procedure as it considers proper, consistent with preserving Peska’s rights of due process. [602 F.2d at 974.]
See also Vandenberg v. Reynolds, 44 CCPA 873, 242 F.2d 761, 113 USPQ 275 (1957).
In resolving the issue of best mode fulfillment, it is immaterial to this court whether the board refers the matter back to the primary examiner or makes the initial determination itself. The board would, in any event, not be bound by the primary examiner’s conclusion that the interference should proceed if in the judgment of the *63board it must be dissolved. Nitz v. Ehrenreich, supra. What is important is that the issue be resolved in the course of the inter partes proceeding when the matter has first been properly raised. Vandenberg v. Reynolds, 44 CCPA at 879, 242 F.2d at 765, 113 USPQ at 278; see also Judge Lane’s dissent in Techler v. Norstrud, 475 F.2d 1192, 1196, 177 UPQ 390, 393 (CCPA 1973).
From the foregoing discussion it is apparent that denominating a matter ancillary to priority does not determine what disposition will be made of the interference. We do not agree with Tofe’s position that the holding in Weil v. Fritz, supra, mandates an award of priority to Tofe if the Winchell application is defective. In Weil v. Fritz, there was no question that the parties both had complete applications and that a valid interference proceeding had been set up over which the board had jurisdiction. In Weil v. Fritz, the issue of best mode fulfillment related to the burden of proof, not to the standing of a party. Fritz sought to prove a priority date in the interference by reliance on an earlier application and, under 35 U.S.C. § 12012, had to have an earlier application in full compliance with 35 U.S.C. § 112. There was no challenge to the adequacies of the applications which were the subject of the interference. An award of priority to one of the parties in that case was, thus, appropriate whether or not the earlier application was acceptable to establish a priority date.
In any event, it would create an anomaly to make the question of best mode satisfaction one to be determined by the board where the application is one of a series, which would be the case if Winchell were to refile, but not ancillary where the application stands alone. However, the resulting disposition need not be the same.
A consideration which may have influenced the encompassing of more and more issues within the “ancillary to priority” rubric 13 is the concern that a contrary holding would preclude an interference party from obtaining an appealable decision on the issue. That concern would obviously be remedied if the board followed a routine practice of referring motions under 37 CFR 1.231(a), absent frivolity or unjustifiable belatedness, to the primary examiner. This court has on a number of occasions expressed its unwillingness to sustain an award of priority of an apparently unpat-entable invention. Cf. Loshbough v. Allen, 54 CCPA 1113, 373 F.2d 747, 152 USPQ 812 (1967).
As a final matter, we endorse the procedure by the board indicated by the report of the proceedings in Steierman v. Connelly, 192 USPQ 433 (Bd.Pat.Intf.1975), modified, 192 USPQ 446 (Bd.Pat.Intf.1976), in which the board made a decision on the ancillary issue of fraud and a decision on the issue of date priority but for the issue of fraud.14 By following this practice, the board will enable this court to address all facets of the issue of priority, should this matter be appealed.
CONCLUSION
Accordingly, we vacate the board’s award of priority to Winchell and remand the case for further proceedings consistent with our opinion herein.
VACATED AND REMANDED
An application for patent for an invention disclosed in the manner provided by the first paragraph of section 112 of this title in an application previously filed in the United States, ... by the same inventor, shall have the same effect, as to such invention, as though filed on the date of the prior application ____ [Emphasis added.]