Special bond structures are the municipal securities bearing special security structures. They are of two types - insured bonds and pre-refunded bonds.
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Insured Bonds: Insured bonds are the bonds secured by the issuer's revenue as well as insurance policies written by the commercial insurance companies. Insurance on a municipal bond is nothing but an agreement by the insurance company to pay the bondholder the amount due on a stated maturity if the issuer defaults on the issue. Once issued, the insurance company cannot cancel its contract for the entire life of the municipal bond.
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Pre-refunded Municipal Bonds: Bonds originally issued either as revenue bonds or general obligation bonds and later pre-refunded by the issuers are called pre-refunded municipal bonds. Pre-refunding normally occurs when the original bonds are escrowed or collateralized by direct obligations guaranteed by the US government.
For this purpose, a trust is created and all the securities guaranteed by the US government are placed into it. In the trust, the securities are arranged in a manner that cash-flows from these securities match the issuer's obligations to pay. When these cash-flows match with the issuer's obligations to pay, then the pre-refunded bonds no longer are secured as the general obligations or revenue bonds. The reason is - these bonds are secured by the cash flows held in the escrow account. This matching of cash flows with the issuer's obligation renders these municipal bonds with less credit risk and makes them the safest municipal bonds.