The 30-year U.S. Treasury Bond. The long bond is so called because it is the bond with the longest maturity issued by the U.S. Treasury. It pays interest semi-annually, and is the most actively traded bond in the world. Because it is backed by the full might of the U.S Treasury and has a low default risk, the long bond is considered by market participants as one of the safest securities around. It is thus considered as a proxy for a “risk-free rate” of investing.