Mass. Gen. Laws ch. 36 § 5
The registers of deeds for Suffolk county and the southern district of Middlesex may, by a writing recorded in their respective registries, appoint one of the persons employed in their respective registries as second assistant register, one of such persons as third assistant register and one of such persons as fourth assistant register, all of whom shall serve at the pleasure of such register; provided, however, that any person so appointed by the register of deeds for Suffolk county who has served continuously in any such position for a period of at least ten years shall not be removed therefrom except for cause.
The register of deeds for the southern district of Middlesex may, by a writing recorded in his registry, appoint one of the persons employed in said registry as fifth assistant register and one of such persons as sixth assistant register.
Mass. Gen. Laws ch. 36, § 5