After a bit of Googling I found out that the Uruguayan football league is very complicated indeed

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The structure of the Uruguayan Championship is extremely complicated, and split into three phases. Firstly, the 18 teams play each other once, with the final table being split into three groups. The top two from each group, plus the remaining four best overall go on to compete in the 10 team Apertura and Clausura championships. This ensures that at least two clubs from the interior qualify for this phase. The Apertura and Clausura winners play-off for the championship, to qualify for the Copa Libertadores, along with the winners of the first phase. The play-off losers then join the three next best placed teams from the overall table in a mini-league or "liguilla" to determine the final Libertadores place.
The eight worst teams from the first phase play in the "Torneo Permanencia" to retain top division status. This is divided into a Montevideo (2 relegated) and an Interior (1 relegated) group, to ensure that 5 clubs from the provinces remain at the top level. Division Two is similarly complicated but results in two Montevideo clubs and one provincial club being promoted, although the best placed provincial club will have to play-off against its relegated counterpart if it finishes lower than 5th. There is no automatic relegation to the third level, which is a small amateur league in Montevideo.
Over the last few years it seems Nacional and Penarol win the top league, Nacional the last three times.
Great site for league structures etc