whom Mr. Justice Clark and Mr. Justice-Harlan join, dissenting.

• English courts. would decide the case as it is being decided here. They would do so because English courts do not recognize the relevance of legislative explanations of the meaning of a statute made in the course of its enactment. If Parliament desires to put a gloss on the meaning of ordinary language, it must incorporate it in the text of legislation. See Plucknett, A Concise History of the Common Law (5th ed.), 330-336; Amos, The Interpretation of Statutes,. 5 Camb. L. J. 163; Davies, The Interpretation of Statutes, 35 Col. L. Rev. 519; Lord Haldane in Viscountess Rhondda’s Claim, [ 1922] 2 A. C. 339, 383-384. Quite otherwise has been the process of statutory construction practiced by this Court over the decades in scores and scores of cases. Congress can be the glossator of the words it legislatively uses either by writing its desired meaning, however odd, into the text of its enactment, or by a contemporaneously authoritative explanation accompanying a statute. The most authoritative form of such explanation is a congressional report defining the scope and meaning of proposed legislation. The most authoritative report is a Conference Report acted upon by-both Houses and therefore unequivocally representing the wilLof both Houses as the joint legislative body.

No doubt to find failure to .file' a declaration of estimated income to be a “substantial underestimate” would be to attribute to Congress a most unlikely meaning for that; phrase in § 294 (d) (2) simpliciter. But if Congress chooses by appropriate means for expressing its . *95purpose to use language with an unlikely and even odd meaning, it is not for this Court to frustrate its purpose. The Court’s task is to construe not English but congressional English. Our problem is not what do ordinary English words mean, but what did Congress mean them to mean. “It is~said that when the meaning of language is plain we are not to resort to evidence in order to raise doubts. That is rather an axiom of experience than a rule of law, and does not preclude consideration of persuasive evidence if it exists.” Boston Sand & Gravel Co. v. United States, 278 U. S. 41, 48.

Here we have the most persuasive kind of evidence that Congress did not mean the language in controversy, however plain it may be to the ordinary user of English, to have the ordinary meaning. íhese provisions were first énaeted in the Current Tax Payment Act of 1943, c. 120, 57 Stat. 126, as additions to § 294 (a) of the Internal Revenue Code of 1939. The Conference Report, H. R. Conf. Rep. No. 510, p. 56, and the Senate Report, S. Rep. No. 221, p. 42, both gave the.provision dealing with substantial underestimation, of taxes the following gloss:

“In the event.of a failure to file any declaration where one is due, the amount of the estimated tax for the purposes of this provision will be zero.”

The revision of the section eight months later by the Revenue Act of 1943, c. 63, 58 Stat. 21, did not affect its substance, and this provision, therefore, continued to carry the original gloss. While the Court adverts to this congressional definition, it disregards its controlling significance.*

*96I agree with the construction placed upon the provision by the 'Third, Fifth, aiid 'Ninth Circuits. Abbott v. Commissioner, 258 F. 2d 537 (C. A. 3d Cir. 1958) ; Patchen v. Commissioner, 258 F. 2d 544 (C. A. 5th Cir. 1958); Hansen v. Commissioner, 258 F. 2d 585 (C. A. 9th Cir. 1958).

delivered the opinion óf the Court.

This, case presents the question whether, under the Internal Revenue Code of 1939, the failure of a taxpayer to file a declaration of estimated income tax, as required by § 58,1 not only subjects him to the addition to the tax *88prescribed by § 294 (d) (1) (A) for failure to file the declaration, but also subjects bim to the further addition to the tax prescribed by § 294 (d) (2) for the filing of a “substantial underestimate’-’ of his tax.

Section 294 (d)(1)(A)' provides, in substance, ■ that if a taxpayer fails to make and. file “a declaration of estimated tax,” within the time prescribed, there shall be added to the tax an amount equal to 5% of each installment due and unpaid, plus 1% of such unpaid install--ments for each month except the first, not exceeding an aggregate of 10% of such unpaid installments.2

Section 294 (d)(2), in pertinent part, provides:

“(2) Substantial underestimate of estimated tax.

“If 80 per centum of the tax (determined without regard to the credits under sections 32 and 35) . . . exceeds the estimated tax (increased by such credits), there shall be added to the tax an amount equal to *89' such excess, or equal to 6 per centum of the amount by which such tax so determined exceeds the estimated tax so increased, whichever is the lesser. . . .” 26 U. S. C. (1952 ed.) § 294 (d) (2). •

Section 29.294-1 (b) (3) (A) of Treasury Regulation 111, promulgated under the Internal Revenue Code of 1939, contains the statement that:

“In the event of a failure to file the required declaration, the amount of the estimated tax for the purposes of [§ 294 (d)(2)] is zero.”

Respondent, without reasonable cause, failed to file a declaration pf his estimated income tax for any of the years 1947 through 1950. The Commissioner imposed an addition to the tax for each of those years under §294 (d)(1)(A) for failure to file the declaration, and also imposed a further addition to the tax for each of those years under §294 (d)(2) for a “substantial underestimate” of the tax. The Tax Court sustained the Commissioner’s imposition of both additions. The Court.of Appeals affirmed with respect to the addition imposed for failure to file the declaration, but reversed with respect to the addition imposed for substantial underestimation of the tax, holding that § 294 (d) (2) does not authorize the treatment, of a taxpayer’s failure to file a declaration of estimated tax as the equivalent of a declaration estimating no tax, and that the regulation, which purports to do so, is not supported by. the statute and is invalid. 258 F. 2d 568. Because of a conflict among the circuits 3 we *90granted the Commissioner’s petition for certiorari. 358 U. S. 940.

The first and-primary question that we must decide is whether there.is any expressed or necessarily implied provision or language in § 294 (d)(2) which authorizes the *91treatment of a taxpayer’s failure to file a declaration of estimated tax as, or the equivalent of, a declaration estimating his tax to be zero.

We are here concerned with a taxing Act which imposes a penalty.4 The law is settled that "penal statutes are to be construed strictly,” Federal Communications Comm’n v. American Broadcasting Co., 347 U. S. 284, 296, and that one “is not to be subjected to a penalty unless the words of the statute plainly impose it,” Keppel v. Tiffin Savings Bank, 197 U. S. 356, 362. See, e. g., Tiffany v. National Bank of Missouri, 18 Wall. 409, 410; Elliott v. Railroad Co., 99 U. S. 573, 576.

Viewing § 294 (d) (2) in the light of this rule, we fail to find any expressed or necessarily implied provision or language that purports to authorize the treatment of a taxpayer’s failure to file a declaration of estimated tax as, or the equivalent of, a declaration estimating his tax to be zero. This section contains no words or language *92to that effect, and its implications look the other way. By twice mentioning, and predicating its application upon, "the. estimated tax” the section seems necessarily to contemplate, arid to apply only to, cases in which a declaration of “the estimated tax” has been made and filed. The fact that the section contains no basis or means for the computation of any addition to the tax in a case where no declaration has been filed would seem to settle the point beyond all controversy. If the section had in any appropriate words conveyed the thought expressed by the regulation it would thereby have clearly authorized the Commissioner to treat the taxpayer’s failure to file a declaration as the equivalent of a declaration estimating his tax at zero and, hence, as constituting a “substantial underestimate” of his tax. But the sectic a contains nothing to that effect, and, therefore, to uphold this addition to the tax would be to hold that it may be imposed by regulation, which, of course, the law does not permit. United States v. Calamaro, 354 U. S. 351, 359; Koshland v. Helvering, 298 U. S. 441, 446-447; Manhattan Co. v. Commissioner, 297 U. S. 129, 134.

The Commissioner points to the fact that both the Senate Report5 which accompanied the bill that became the Current Tax Payment Act of 1943,6 and the Conference Report7 relating to that bill, contained the statement which was later embodied in the regulation. He then argues that by reading § 294 (d) (2) in connection with that statement in those reports it becomes evident *93that Congress intended by § 294 (d) (2) to treat the failure to file a declaration as the equivalent of a declaration estimating no tax. He urges us to give effect to the congressional intention which he thinks is thus disclosed. However, these reports pertained to the forerunner of the section with which we are now confronted, and not to that section itself. Bearing in mind that we are here con--cerned with an attempt to justify the imposition of a second penalty for the same omission for which Congress has specifically provided a separate and very substantial penalty, we cannot say that the legislative history of the initial enactment is so persuasive as to overcome the language of § 294 (d)(2) which seems clearly to contemplate the filing of an estimate before there can be an underestimate.

The Commissioner next argues that the fact that Con-, gress, with knowledge of the regulation, several times amended the 1939 Code but left § 294 (d) (2) unchanged, shows that Congress approved the regulation, and that we should accordingly hold it to be valid. This argument is. not persuasive, for it must bé presumed that Congress also knew that the courts, except the Tax Court, had almost uniformly held that § 294,(d) (2) does not authorize an addition to the tax in a case where no declaration has been filed, and that the regulation is invalid.8 But the point is immaterial, for Congress could not add to or expand this statute by impliedly approving the regulation.

These considerations compel us to conclude that § 294 (d) (2) does not authorize the treatment of a taxpayer’s failure to file a declaration of estimated tax as the equivalent of a declaration estimating his tax to be zero. The questioned regulation must therefore be regarded “as *94no more than an attempted addition to the . statute of something which is not there.” United States v. Calamaro, supra, 354 U. S., at 359.

Affirmed.