Most commonly the music is tango, milonga or vals.
Between tandas is played a cortina (Spanish for "curtains"), a musical pause to allow dancers to leave the floor and to serve as a short break between tandas.
It is common to choose the music for a tanda from a given orchestra and era, often with the same vocalist (or, as an alternative, purely instrumental pieces).
Mixing different vocalists, eras or even orchestras is rare.
This approach is chosen for the comfort of the dancers, with two main reasons [1] Sometimes, in a minority of milongas in Argentina, non-tango tandas are also played, like Rock n roll, Cumbia or Chacarera, but rarely for more than one tanda.