This action involves the interpretation of a written contract extending the enlistment of Petitioner, a Navy enlistee, who had, on April 25, 1966, signed a written enlistment (Ex. B) containing the language noted in the footnote.

After initial processing, Petitioner executed a contract (Ex. C) extending the enlistment for two years, the relevant provisions of which are as follows:

Petitioner completed four years of naval service and filed a petition for a writ of habeas corpus to compel his discharge. His contention is that he had completed only one Class A school, that is, the MM Class A school in Great Lakes, Illinois. He argued that the agreement is clear and unambiguous and required his completion of three Class A schools, IC (Interior Communications), EM (Electrician’s Mate) and MM (Machinist’s Mate) as a condition precedent to the two-year extension of his enlistment. The District Court agreed and, in invoking the parol evidence rule, sustained Petitioner’s objection to the reception in evidence of BUPERS INSTRUCTION 1306.64E as an aid in the interpretation of the agreement. This is a long directive issued by the Active Chief of Naval Personnel dealing with information concerning Nuclear Field Personnel. While it is relevant in its entirety, two extracts thereform will serve to demonstrate this :

The Government argues first that the Bupers Instruction 1306.64E cannot be ignored because it has the force and effect of law which is automatically a part of the contract. It is well settled that existing laws are read into contracts in order to fix the rights and obligations of the parties. Home Building and Loan Association v. Blaisdell, 290 U.S. 398 , 54 S.Ct. 231 , 78 L.Ed. 413 (1934); Federal Crop Ins. Corp. v. Merrill, 332 U.S. 380 , 68 S.Ct. 1 , 92 L.Ed. 10 (1947). This is also true of valid regulations having the force and effect of laws of general application.

The extension of enlistment agreement was authorized by statute. 10 U.S.Code, 1964 Ed., p. 1515, § 5539(a):

Not only does Section 5539(a) authorize extension of enlistment agreements under regulations prescribed by the Secretary of the Navy with the approval of the President, but Section 6011 of Title 10, United States Code, also provides specifically: “United States Navy Regulations shall be issued by the Secretary of the Navy with the approval of the President.” The regulations so issued have the force of law. Cafeteria & Restaurant Workers Union, Local 473, A.F.L.-C.I.O. v. McElroy, 367 U.S. 886 , 81 S.Ct. 1473 , 6 L.Ed.2d 1230 (1961).

The United States Navy Regulations 1948, in effect in 1966, were issued by the Secretary of the Navy and approved by President Truman on August 9, 1948. Section 1201, “Regulatory Publications for the Department of the Navy,” provides, in pertinent part:

The Bupers Instruction 1306.64E which was rejected as evidence and disregarded as irrelvant to the interpretation of the extension of enlistment agreement was issued by the Acting Chief of Naval Personnel on April 13, 1966. We conclude that it was part of the governing law in effect when the agreement was executed and could not be disregarded in the interpretation of the agreement. The offer of the regulation as evidence was merely for the convenience of the trial court. It was, in legal effect, “in evidence” without such offer.

We believe, further, that the District Court erred in rejecting Bupers Instruction 1306.64 as incompetent parol evidence. Even an ancient view of the parol evidence rule, which, in its classical expression, precludes the reception of parol evidence to vary, contradict or add to the terms of a written instrument, permits evidence of the circumstances of the parties and the associated surroundings so that meaning may be given to the words used. For instance, in Reed v. Insurance Co., 95 U.S. 23, at 30 , 24 L.Ed. 348 (1877), the Court said:

See also Porto Rico Sugar Company v. Lorenzo, 222 U.S. 481 , 32 S.Ct. 133 , 56 L.Ed. 277 (1912).

In the instant case, the extension of enlistment agreement expressly refers to “Training (Nuclear Field Program — Bupersinst 1306.64 Series * * *, obviously, it seems to us, as a reference for a more complete description of the scope and purposes of the training contemplated. A reference in a written agreement to an extraneous writing makes it part of the agreement, at least for the purposes indicated. Guerini Stone Co. v. P. J. Carlin, 240 U.S. 264 , 36 S.Ct. 300 , 60 L.Ed. 636 (1916).

We conclude, for the reasons stated, that the trial court erred in rejecting BUPERS INSTRUCTION 1306.64 as a competent and material aid in interpretation of the language of the agreement.

The judgment of the District Court is reversed and the case is remanded for further proceedings consistent with this opinion.

. “I understand that I am enlisting under the provisions of the High School/Junior College Graduate Training Program in the rate of SA-JCNF. The provisions of this program and the category in which I am enlisting have been fully explained to me. I understand the final determination as to the school or course of Training to which I am assigned will be determined subsequent to enlistment and will be dependent on the following: Rremaining (sic) qualified by my performance both military and professional. Satisfactory completion of recruit training. The overall needs of the Navy. Results of basic battery test scores. Results of special aptitude tests. Physical qualifications, including qualifications (sic) for submarine duty if enlisting in Nuclear or Polaris Fields. Where compatible, my personal desires. I certify that no promise as to the specific school or course of Training to which I will be assigned has been made to me and that I further understand that although I was found eligible for enlistment and general service and I meet the requirements for the SA-JCNF category of the HS/JCGTP, I may later be found ineligible to continue in this category because of my eligibility for security clearance. I also understand that should such a situation occur, I may be assigned to another category for the HS/JCGTP for which I am qualified. ROBIN ANDREW REHART APPLICANT’S SIGNATURE WITNESSED: Larry L. Claton LARRY L. CLATON, 2nd LT, AGO ASSISTANT ADJUTANT “DATE. APR 25 1966 I have been personally interviewed by the Commanding Officer or a Recruiting Officer and have had all provisions of the program under which I am enlisting explained to me, including the six year service obligation to be eligible for Nuclear Power School. No promise has been made regarding assignment to either submarine or surface duty. ROBIN ANDREW REHART APPLICANT’S SIGNATURE WITNESSED: LARRY L. CLATON LARRY L. CLATON, 2nd LT, AGC ASSISTANT ADJUTANT” . This is a federal contract governed by federal law. In so limiting our discussion of the parol evidence rule as applied to federal contracts, we intend no implication with respect to the adoption or rejection by the federal courts of the more liberal interpretation of the rule espoused by Professor Corbin, 3 Corbin on Contracts, Chapter 26, § 573, et seq. Cf. Restatement of Contracts, §§ 230, 231; Masterson v. Sine, 68 Cal.2d 222 , 65 Cal. Rptr. 545 , 436 P.2d 561 (1968).