North Carolina Administrative Code Title 17 — Revenue
17 NCAC 05C .1402 — Tax-Exempt Bonds
The amount of premium paid upon the purchase of a tax-exempt bond is amortized over the life of the bond. Amortization for the taxable year is accomplished by reducing the original cost of the bond by a portion of the premium paid, with no deduction against net income for the year. Therefore, when the bond is sold or otherwise disposed of, the basis for determining gain or loss will always be original cost less the amount of premium amortized for book purposes through the year of disposal.
Source: official text