26 U.S.C. § 872: Gross income

gross income which is derived from sources within the United States and which is not effectively connected with the conduct of a trade or business within the United States, and gross income which is effectively connected with the conduct of a trade or business within the United States. Gross income derived by an individual resident of a foreign country from the international operation of a ship or ships if such foreign country grants an equivalent exemption to individual residents of the United States. Gross income derived by an individual resident of a foreign country from the international operation of aircraft if such foreign country grants an equivalent exemption to individual residents of the United States. a nonresident alien individual, foreign partnership, or foreign corporation, or an office or place of business maintained in a foreign country or in a possession of the United States by a domestic corporation, a domestic partnership, or an individual who is a citizen or resident of the United States. Income derived by a nonresident alien individual from a series E or series H United States savings bond, if such individual acquired such bond while a resident of the Ryukyu Islands or the Trust Territory of the Pacific Islands. Gross income derived by a nonresident alien individual from a legal wagering transaction initiated outside the United States in a parimutuel pool with respect to a live horse race or dog race in the United States. Income to which paragraphs (1) and (2) apply shall include income which is derived from the rental on a full or bareboat basis of a ship or ships or aircraft, as the case may be. The Secretary may provide that this subsection be applied separately with respect to income from different types of transportation. To the extent provided in regulations, a possession of the United States shall be treated as a foreign country for purposes of this subsection.