A tanda is a turn of dancing in a milonga, and by association, a set of pieces of music, usually between three and five, that is played for one turn. The most common style is to play four pieces in the tango tandas, three in the milonga tandas, and three or four in the vals tandas.
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.
Tandas are normally arranged by feel of the music. A typical order of tandas is: T-T-V-T-T-M (T for a tanda of tangos, V for a tanda of vals cruzado, M for a tanda of milongas).
Structure inside a tanda
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
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.
To separate one tanda from the next, a short piece of non-tango music called a 'cortina' (curtain) can be played, as a signal to clear the floor. This is the main reason for the existence of a tanda - it gives an opportunity to select your next partner from the whole group of dancers present: if three tracks are played without the cortina, only one third of the dancers are available at the end of each track.