After each Clausura tournament (May/June, end of the season), there are 2 relegated and 2 promotion teams.
In Argentina relegations are decided using a special table called ”Averages” where the 20 teams are ordered by their last 3 seasons (Apertura + Clausura) points added, divided by the number of games played.
This system has been in use for 25 years, and has been always very criticized because it favours bigger teams (they can have one or two bad tournaments, but they will rarely have more than 3 or 4 tournaments with numbers bad enough to end in the lower spots of this table) and makes it more difficult for teams recently promoted.
These teams need to have an above average season (45-50 points adding Apertura + Clausura) to stay in 1st division because their average is calculated with less games and so it’s difficult for them to keep a good average.
The 19th and 20th teams in this table are relegated to 2nd division, and the 17th and 18th teams play a promotion playoff against a 2nd division team each (home and away games, if they have the same points and goal difference after this two games, the 1st division team stays).
This is how the Averages Table will start for the 2010/2011 season (click on the image to see it bigger).
As you can see, Olimpo, Quilmes and All Boys start from 0 (zero) because they are the newly promoted teams (All Boys did it through one of the promotion playoffs), and for the first time ever, one of the big clubs of Argentina, River Plate, starts in a very dangerous situation.