http://www.verytangostore.com/tango-milonga.html

"The Definition of 'Milonga'

The milonga has the following attributes:

• A fast-paced dance which originated in Buenos Aires, and preceded modern tango.
• It incorporates many of the steps in tango, but due to its pace, the vast repertoire available to modern tango cannot be reproduced in full in the milonga (for example many of the embellishments, which require a pause).
• It is not a true tango in the very strictest sense as rather than being in the 4/4 standard tango pattern, it is 2/4:"

http://www.totaltango.com/acatalog/tango_milonga_vals_92.html

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