A bond index or bond market index is a method of measuring the value of a section of the bond market. It is computed from the prices of selected bonds (typically a weighted average). It is a tool used by investors and financial managers to describe the market, and to compare the return on specific investments.An index is a mathematical construct, so it may not be invested in directly. But many mutual funds and exchange-traded funds attempt to "track" an index (see index fund), and those funds that do not may be judged against those that do.