Certificates of indebtedness representing long-term borrowing of capital funds, the terms of which contain an indenture pledging the property as security for the loan and providing for the appointment of a trustee to represent the bondholders. If the lien of the mortgage is limited to specific property owned at the time the mortgage was created and to replacements thereof, the mortgage is described as "closed." If the lien extends to "after acquired" property which may be used as the basis for issuance of additional bonds under the terms and provisions of the indenture, the mortgage is referred to as an "open-end" mortgage.