The final tranche in a series of mortgage-backed securities that is the last one to receive payment. Used in some collateralized mortgage obligations (CMO), Z-bonds pay no coupon payments while principal is being paid on earlier bonds. Interest that would have been paid on Z-bonds is used instead to pay down principal more rapidly on the earlier series of bonds. 



|||Interest payable on a Z-bond is added to the principal balance and becomes payable once claims on all prior bond classes have been satisfied. A Z-bond is similar to a zero-coupon bond, since it accrues interest rather than paying it out. Therefore, the final tranche is considered the most risky for the CMO class structures.