Sur is a tango with music by Aníbal Troilo and lyrics by Homero Manzi. It was first recorded by Troilo's orchestra with vocals by Edmundo Rivero on 23 February 1948. The first live performance, by the same artists, was at the Tibidabo night club in Buenos Aires.
Sur (tango) Wikipedia
The song is an elegy for a lost love, framed in the landmarks of the South side of Buenos Aires, lamenting both the end of a love story and the changes in the barrio. The male narrator addresses the girl in the second person; it is mentioned that the girl was 20 at the time.
Among the landmarks mentioned are: the corner of San Juan and Boedo at the center of the Boedo neighborhood, Pompeya (located directly to the South of Boedo), the railway crossing and the swampland at the (southern) edge of Pompeya, and the enigmatic "blacksmith's corner, mud and pampa", which could refer to the corner of Centenera and Tabaré, already named in Manzi's earlier "Manoblanca"  or to a blacksmith shop in the corner of Inclán and Loria, in Parque Patricios neighbourhood.
Manzi himself was born in Añatuya, Santiago del Estero, and moved into Buenos Aires at the age of nine, living close to the landmarks mentioned in the tango.
Rivero himself made two small changes to the lyrics, with Manzi's blessing: : "florando" became "flotando" ("flowering" to "floating", as the original verb is uncommon and was not understood by audiences), and "y mi amor y tu ventana" became "y mi amor en tu ventana" ("and my love and your window" became "and my love in your window"). The first of these changes was universally adopted.
Troilo's collaboration with Manzi yielded several hits during the 1940s, including "Barrio de Tango" and the waltz "Romance de Barrio" , but none achieved the universal recognition of "Sur", perhaps the tango most loved by Argentines, and certainly one of the most conspicuously recorded .
Besides Rivero's original recording, notable versions include covers by Julio Sosa, Nelly Omar, Roberto Goyeneche, and Andrés Calamaro.
Argentine author Ernesto Sabato has said  that he'd give away all he's written for the privilege of being the author of "Sur".