Primera División (first division) is the top category of Argentine football (soccer), and it's organized by the Argentine Football Association. Currently, there are 20 teams in the first division.
The 20 teams play two single-round tournaments each year: the Clausura (February to June), and the Apertura
(August to December). Thus, there are two champions each season. Unlike
most European countries, Argentina has no official Cup competition.
The names of the tournaments (Closing and Opening, respectively) reflect Northern Hemisphere sports seasons. This scheme was introduced in the 1980s, and is alien to Argentines - who live in the Southern Hemisphere and are used to sports seasons that span a single calendar year (for example 2004, as opposed to 2004/05).
Relegation is based on an averaging
system. At the end of each season, the two teams with the worst
three-year averages are relegated, and the best two teams in the second division
are promoted. The teams placed 17th and 18th in the averages table play
a best-of-three playoff (called "promoción") with the 4th and 3rd
second division teams, respectively. Thus, the number of teams promoted
each year varies between two and four. Newly-promoted teams only
average the seasons since their last promotion.
It is commonly believed that averaging was instituted in the 1980s to minimize the chance of large clubs being relegated, after San Lorenzo de Almagro was relegated in 1981, Racing Club was teetering on the brink, and Boca Juniors had a dismal 1984 season.