In USA there are two types of Municipal Bonds depending upon origin.
One is issued by a particular state is called as in-house bond and those issued by other states wrt that state is called out-of-state bonds.
Generally the in –house bonds are provided a status by the state under which tax (federal or state) is not charged on the interest part.
These bonds are very good options for taxpayers intending to minimize taxes on interest income. Thus the two process of tax exemption and interest benefits makes these bonds as double exempt bonds.