26 C.F.R. § 1.103-1 : Internal Revenue — INTERNAL REVENUE SERVICE, DEPARTMENT OF THE TREASURY — INCOME TAX — INCOME TAXES —COMPUTATION OF TAXABLE INCOME —ITEMS SPECIFICALLY EXCLUDED FROM GROSS INCOME —1 - Interest upon obligations of a State, territory, etc

(a) Interest upon obligations of a State, territory, a possession of the United States, the District of Columbia, or any political subdivision thereof (hereinafter collectively or individually referred to as "State or local governmental unit") is not includable in gross income, except as provided under section 103 (c) and (d) and the regulations thereunder.
(b) Obligations issued by or on behalf of any State or local governmental unit by constituted authorities empowered to issue such obligations are the obligations of such a unit. However, section 103(a)(1) and this section do not apply to industrial development bonds except as otherwise provided in section 103(c). See section 103(c) and §§1.103-7 through 1.103-12 for the rules concerning interest paid on industrial development bonds. See section 103(d) for rules concerning interest paid on arbitrage bonds. Certificates issued by a political subdivision for public improvements (such as sewers, sidewalks, streets, etc.) which are evidence of special assessments against specific property, which assessments become a lien against such property and which the political subdivision is required to enforce, are, for purposes of this section, obligations of the political subdivision even though the obligations are to be satisfied out of special funds and not out of general funds or taxes. The term "political subdivision", for purposes of this section denotes any division of any State or local governmental unit which is a municipal corporation or which has been delegated the right to exercise part of the sovereign power of the unit. As thus defined, a political subdivision of any State or local governmental unit may or may not, for purposes of this section, include special assessment districts so created, such as road, water, sewer, gas, light, reclamation, drainage, irrigation, levee, school, harbor, port improvement, and similar districts and divisions of any such unit.

26 C.F.R. §1.103-1

T.D. 7199, 37 FR 15486, 8/3/1972