Tandas and cortinas form the basic structure of the milonga but they are more than just the paragraphs and commas of the event. They bring order on many different levels. Firstly, we know that the tango is an intimate dance and the structure enables couples who are not necessarily in love to come together and dance the dance of love for ten minutes only. The dance is the dance only and however intimate the tango may look, the intimacy of the moment is usually forgotten once the partners separate. On to the next tanda.... However, if couples dance more than a couple of tandas together in one night and they are not romantically involved, it would usually generate a bit of interest in those observing, and certainly those who are dancing. Ladies, the bottom line is that if you want to go home alone at the end of the night don't dance a whole batch of tandas with the same guy. You will be giving him the wrong message. In Buenos Aires, the wrong message is very difficult to convert to the right message and you might find yourself with a sulky friend and a little nasty comment to take home with you instead.
Artisitically speaking, the tanda format provides the tango DJ with the opportunity to arrange the music according to tango,walz and milonga and according to specific orchestras. At Milonga Africana, for example, all tandas contain pieces from the same orchestra of the Golden Age of tango e.g. D'arienzo, Calo, Canaro. This is intended to create an element of appreciation for the orchestra, continuity and depth to the tanda. It also provides dancers the opportunity to get into the mood for a tanda and escape into it completely.
You may be interested to know that the history of this sensible and dignified tradition has less than salubrious origins. Tango arose from the steaming streets and bordellos of Buenos Aires. Far from an elegant and cultured dance, it was a dance of prostitutes and lonely men ( Only once it was taken to Europe, did it gain the approval of the chattering classes). These were the immigrants and descendants of slaves that lived at the mouth of the Rio de la Plata at the end of the 1800s. In dance halls, men arrived alone and paid for women to dance with them. They bought a coupon made of tin called a "lata". The lata entitled them to dance with the lady for three songs (the tanda). A man would need to get another coupon if he wanted to dance with the lady again.
Tips for dancing tandas in a tradtional setting:
1. If you have never danced with a lady before, have a little "charla" (chat) before you embrace her. This helps to relax the couple.
2. It is customary to separate during a cortina so if you ask a lady to dance in the last song of a tanda, it may be a short dance.
3. Respect the cortina by separating and moving off the dance floor, even if it is a song you want to boogie to.
4. It is considered very rude to stop dancing with someone in the middle of a tanda. If you feel really uncomfortable and cant bear it any more, just say "thank you" at the end of the song.