The fund normally invests at least 80% of its net assets (plus any borrowings for investment purposes) in obligations issued by or on behalf of states, territories and possessions of the United States and the District of Columbia and their political subdivisions, agencies or instrumentalities, the interest on which is exempt from regular federal income tax. It normally acquires municipal obligations with maturities of ten years or more, but may acquire municipal obligations with shorter maturities or that are subject to shorter call provisions.