Yield calculation used, in lieu of "Yield to Maturity" or "Yield to Call," where books are retired systematically during the life of the issue, as in the case of a "Sinking Fund," with contractual requirements. Because the issuer will buy its own bonds on the open market to satisfy its sinking fund requirements if the bonds are trading below Par, there is, to that extent, automatic price support for such bonds; they therefore tend to trade on a yield-to-average-life basis.