26 C.F.R. § 521.117
Under Article XX of the convention, where the action of the revenue authorities of the contracting States has resulted in double taxation in respect of any of the taxes to which the convention relates, the taxpayer is entitled to lodge a claim with the country of which he is a citizen or, if he is not a citizen of either country, with the country of which he is a resident, or if the taxpayer is a corporation or other entity, with the country in which it is created or organized. Article XX further provides that should the claim be upheld, the competent authority of the country with which the claim is lodged may come to an agreement with the competent authority of the other country with a view to equitable avoidance of the double taxation. Such a claim on behalf of a United States citizen or corporation or other entity, or on behalf of a resident of the United States who is not a Danish citizen, shall be filed with the Commissioner of Internal Revenue, Washington, D.C. The claim should be set up in the form of a letter and should show fully all facts on the basis of which the claimant alleges that such double taxation has resulted. If the Commissioner of Internal Revenue determines that there is an appropriate basis for the claim under the convention, he will take the matter up with the Chief of the Taxation Department of the Ministry of Finance of Denmark with a view to arranging an agreement of the character contemplated by Article XX.
26 C.F.R. § 521.117