From D'Addario:

True to name, this set was designed to suit the needs of gypsy jazz guitar players in an era before amplification. Made by John D’Addario Sr. for legendary luthier Mario Maccaferri, Gypsy Jazz strings embody the authentic tone and feel of acoustic jazz. Constructed with a high carbon steel core and silver-plated copper wrap wire, this set delivers the beautifully warm, percussive, and projecting tone heard in the recordings of Django Reinhardt and other acoustic jazz ensembles. This 11-45 Medium Loop End set is made for acoustic guitars which use loop end strings and has a bit more tension than light gauge strings.