This book describes the characteristics and applications of the ¿a photometric system, which measures the stellar flux at around 520 nm. Since its introduction in 1976 by Hans-Michael Maitzen, the ¿a system has not only been successfully applied to the classical chemically peculiar stars of the upper main sequence, but also to galactic and extragalactic star clusters. New possible applications for studying galaxies, quasars and other astrophysical objects of interest are discussed.