|This article needs additional citations for verification. (March 2009)|
In sport (particularly association football), a two-legged tie is a contest between two teams which comprises two matches or legs, with each team as the home team in one leg. The winning team is usually determined by aggregate score, the sum of the scores of the two legs. For example, if the scores of the two legs are:
- First leg: Team A 4–1 Team B
- Second leg: Team B 2–1 Team A
then the aggregate score will be Team A 5–3 Team B, meaning team A wins the tie. In some competitions, a tie is considered to be drawn if each team wins one leg, regardless of the aggregate score. Two-legged ties can be used in knockout cup competitions and playoffs.
In North America, where the word "tie" when used in the context of sports normally refers to a game ending in a draw, the term "two-game total-goals series" is the more common usage to refer to this competition format.
In association football, two-legged ties are used in the later stages of many international club tournaments, including the UEFA Champions League and the Copa Libertadores; in many domestic cup competitions, including the Coppa Italia and the Copa del Rey; in domestic league play-offs, including the Football League play-offs and the MLS Cup Playoffs; and in national-team playoffs in some qualification tournaments, including FIFA World Cup qualification.
In ice hockey, the National Hockey League used "two-game total goals series" in the early years of its playoffs. It applied to all its playoffs from 1918 to 1926, and the early rounds until 1937, when it completed the switch to best-of-N series. The NCAA Men's Ice Hockey Championship also used a two game total goals format for much of its history.
In basketball, the two top European club competitions, the Euroleague and Eurocup, both use two-legged ties in the qualifying rounds that determine the clubs advancing to each competition's group phase. The Eurocup also uses two-legged ties in its quarterfinal round, which will be a separate phase of the competition starting in 2009–10. The French Pro A league used two-legged ties in all of its playoff rounds, except for the one-off final, until the 2006–07 season. At that time, all of its playoff rounds leading up to the final, which remained a single match through 2011–12, were changed to best-of-three series. The final changed to best-of-five starting in 2012–13.
If the aggregate score is tied after the two legs, various methods can be used to break ties. Under the away goals rule, the team who scored more away goals advances. If away goals are equal, or are not considered, then the tie may be decided by extra time and/or penalty shootout. Replays, at the second-leg venue or a neutral venue, were formerly used in European club competitions. In the Liguilla (playoffs) of the Primera División de México, the team with the better regular-season record advances. In the promotion playoffs in Italy's Serie B (which do not necessarily occur in a given season), two-legged ties that are level on aggregate at the end of regulation time of the second leg go to extra time (away goals are not used); if the tie remains level after extra time, the team that finished higher on the league table advances.
Second leg home advantage
Each team hosts one match, and there is no intended advantage to whether a team plays at home first or second. However, lots of managers and players believe that the team playing at home for the second leg has a slight advantage over the other team. The thinking is that the team playing away for the first leg can play it safe there (a draw or even a slight defeat is considered a favourable result), and then "win" the tie at home in the second leg. For example, in the draw for the first knockout round of the UEFA Champions League, the group runners-up will play the first leg at home, while the group winners will play the second leg at home. A statistical analysis of roughly 12,000 matches from the European club competitions between 1956 and 2007 showed that around 53% of teams playing at home in the second leg won the tie (even after controlling for the fact that team playing at home in the second leg tend to be better teams). This effect has however decreased over time.
In some competitions, there is an away goals rule that dictates that, if the tie ends drawn, goals scored at the other team's stadium count twice, rather than the usual once.  If still tied, then extra time is played. If tied after that, then, depending on the competition, the away goals rule is reapplied.  This results in an advantage for the second leg hosts, because if the home team scores once, the away team has to score twice.
In knockout competitions, alternatives to two-legged ties include:
- single-leg ties:
- either where one team has home advantage, as in all rounds of the FA Cup except the semi-finals onwards. Two-legged ties are seen as fairer, since they give neither team home advantage; conversely, in the National Football League, home advantage is a reward for being the better seed or, in the opening wild-card round, winning the division.
- or played at a neutral venue, as in the final match of many tournaments, including the UEFA Champions League Final, the FA Cup Semi-finals and Final, and the NFL's Super Bowl. Neutral venues may be inconvenient for a team's fans to travel to, and due to this and the much higher prices for such a marquee event, a championship at a neutral site often draws a crowd of a much different nature than a crowd at a regular season contest.
- best-of-N match series, where the team winning more matches wins the series. These are common in major Canadian and American sports leagues. If a game cannot be drawn and there is an odd number of games in the series, the series cannot be drawn either. A best-of-N series can end in an odd number of games, and such series are typically structured with alternating host sites so that the higher ranked team gets the extra game.
- Ross, James M. (27 June 2007). "Inter-Cities Fairs Cup 1960-61". RSSSF. Retrieved 2009-03-26.
- "The second leg home advantage: Evidence from European football cup competitions.".