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interpretation when it constitutes objective evidence of the understanding of the Parties as to themeaning of the treaty. 125 For example, the subsequent practice of States in the application of theGWS-Sea’s requirements for hospital ships has clarified that States may use hospital ships withthe capability to conduct encrypted communications. 1261.7.5 Treaties and Domestic Implementing Legislation. States may enact domesticlegislation to implement treaty provisions. Although such implementing legislation is notinternational law, it may reflect a State’s interpretation of those provisions. 127A State’s domestic implementing legislation, or lack of such legislation, however, doesnot justify that State’s noncompliance with an international obligation as a matter of internationallaw. 1281.8 CUSTOMARY INTERNATIONAL LAWCustomary international law results from a general and consistent practice of States thatis followed by them from a sense of legal obligation (opinio juris). 129 Customary internationallaw is an unwritten form of law in the sense that it is not created through a written agreement byStates.subsequent agreement between the parties regarding the interpretation of the agreement, and subsequent practicebetween the parties in the application of the agreement, are to be taken into account in its interpretation.”); IRESTATEMENT (THIRD) OF FOREIGN RELATIONS LAW OF THE UNITED STATES 203 (§325, comment c) (1987) (This“conforms to United States modes of interpretation, affirming that subsequent practice of the parties can be takeninto account in interpreting international agreements.”).125 See II YEARBOOK OF THE INTERNATIONAL LAW COMMISSION 221 (15) (1966) (“The importance of suchsubsequent practice in the application of the treaty, as an element of interpretation, is obvious; for it constitutesobjective evidence of the understanding of the parties as to the meaning of the treaty. Recourse to it as a means ofinterpretation is well-established in the jurisprudence of international tribunals.”). See also Case ConcerningKasikili/Sedudu Island (Botswana v. Namibia), Judgment, 1999 I.C.J. 1045, 1075-76 (49) (same); Russian Claimfor Interest on Indemnities (The Russian Indemnity Case), Russia/Turkey, 11 R.I.A.A. 421, 433 (1912) PermanentCourt of Arbitration Unofficial English Translation, 3 (“Whereas the fulfilment of obligations is, between States asbetween individuals, the surest commentary on the meaning of these obligations;”); Case Concerning The Temple ofPreah Vihear (Cambodia v. Thailand), Merits, Judgment, 1962 I.C.J. 6, 34 (The map “was accepted by the Parties in1908 and thereafter as constituting the result of the interpretation given by the two Governments to the delimitationwhich the Treaty itself required. In other words, the Parties at that time adopted an interpretation of the treatysettlement which caused the map line, in so far as it may have departed from the line of the watershed, to prevailover the relevant clause of the treaty.”).126 Refer to § 7.12.2.7 (Use of Secret Codes for Communication).127 See United States v. Navarre, 173 U.S. 77, 79 (1899) (noting that “[i]f the meaning of the treaty was doubtful, itwas competent for Congress to resolve the doubt” in its enactment of legislation).128 Refer to § 1.10.1.4 (Force of International Law Notwithstanding a State’s Domestic Law).129 See I RESTATEMENT (THIRD) OF FOREIGN RELATIONS LAW OF THE UNITED STATES 24 (§102(2)) (1987)(“Customary international law results from a general and consistent practice of states followed by them from a senseof legal obligation.”).29