A tanda is a turn of dancing in a milonga, and by association, a set of pieces of music, usually between three or 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 
- When a new tanda starts the dancers can listen to the beginning of the first piece to learn whether they like the music or not, and based on this decide to ask someone to dance this tanda, or take a break.
- The dancers can adjust to a type of music in the beginning tanda, without having to readjust for each song.
- Sedó, Melina. Heroes of the Silver Disc - Guidelines for successful DJing. Tangodanza, 2006 (revised 2008 online)