The teaching of Argentine Tango usually follows a curriculum that is sequential: Tango 1, Tango 2, Tango 3, etc, where each class series is prerequisite for the next. As the student progresses, at a certain point she completes her tango foundation. After this, the subject matter branches out into topics classes of one sort or another such as Rhythm of Vals, or Milonga Traspie, or an element class such as Boleos or Barridas, or whatever. What constitutes the Tango Foundation?