Sinking fund provision? provision, through, related, currently, benefit

suxiaoxiao

I'm going through fix income and schweser notes says, if market rates is currently below the coupon rate, the bonds will be trading at a premium to par value. Thus, a sinking fund call at par would not benefit a bondholder.

can anyone explaine this? not quite sure what a Sinking fund provision really is..I thought it's related to bond issuer, how does it link to bondholders? Thx!