2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/6914We highlight the importance of the MgII emission-line doublet in probing high-redshift quasars and their supermassive black holes. In the SDSS era, where large scale investigations of quasars across the age of the Universe are possible, this emission-line has the ability to provide accurate systemic redshifts which are important for a variety of follow-up studies, as well as probe the masses of the supermassive black holes that power these phenomena.4 pages, 4 figures. To appear in "AGN Physics with the Sloan Digital Sky Survey, ed. G. T. Richards and P. B. Hall (San Francisco: ASP)"Astrophysicstext