Liga MX is the top level of the Mexican football system which was established in 1943. As of 2012 Liga MX had 18 clubs participating in a 2 tournament season. The tournaments are called the Apertura and the Clausura. Each tournament is a 17 week tournament where the top 8 teams move to the Liguilla. The Liguilla is a two-leg aggregate-score. If there is a tie in a Liguilla match after the 2nd game the higher seed moves on to the next round. In the final a tie forces extra time of two 15 minute periods and penalty kicks if needed. Relegation is also used in Liga MX. At the end of each season one team is relegated to Ascenso MX which allows one team from Ascenso MX to be promoted in their place. Relegation is determined by the lowest ratio of points-per-game-played for each team for the past 3 seasons.