A stipulation in many bond indentures that the borrower retire a certain proportion of the debt annually. The retirement may be effected by calling the bonds from the investors (if interest rates have declined) or by purchasing the bonds in the open market (if interest rates have increased). This orderly retirement may be advantageous to a bondholder because it creates some liquidity; however, it also may cause the holder to give up a high-yielding bond at the call price (often at par) during a period of reduced interest rates. Compare doubling option. Also called bond sinking fund. See also funnel sinking fund.
