Can Barcelona do something unprecedented? Who has the statistical edge between Leicester and Sevilla? We look at each team's chances of going through from their current position.
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The eight UEFA Champions League round of 16 first legs produced eight different scorelines, with six of the initially home teams having leads to defend away from home and two needing to overturn deficits.
Scores ranged from 1-0 and 2-1 to an unusual 5-3, and convincing 5-1 and 4-0 victories. But who does history say will go through?
We looked at every past first leg in the European Cup/UEFA Champions League, UEFA Cup/UEFA Europa League and European/UEFA Cup Winners' Cup with an identical scoreline to the current round of 16 ties and noted which teams went though.
So while the precedents say Leicester and Sevilla are virtually level, Arsenal and Bayer Leverkusen must both attempt to do things that only one club has managed in the past, and in Barcelona's case something that has never been achieved. Oh, and if all six 'unseeded' sides hold on and go through, that too will make history ...
Napoli v Real Madrid (1-3)
Times first-leg scoreline has happened in UEFA men's club competition: 329
Home side in second leg through: 75 (22.8%)
Away side in second leg through: 254 (77.2%)
Arsenal v Bayern München (1-5)
Home side in second leg through: 1 (1.2%)
Away side in second leg through: 79 (98.8%)
• The only team to recover from a 5-1 defeat were Real Madrid, who beat Borussia Mönchengladbach 4-0 in the 1985/86 UEFA Cup third round to progress on away goals.
Borussia Dortmund v Benfica (0-1)
Home side in second leg through: 402 (40.6%)
Away side in second leg through: 588 (59.4%)
Barcelona v Paris Saint-Germain (0-4)
Home side in second leg through: 0 (0%)
Away side in second leg through: 58 (100%)
Juventus v Porto (2-0)
Home side in second leg through: 330 (97.3%)
Away side in second leg through: 9 (2.7%)
Leicester City v Sevilla (1-2)
Home side in second leg through: 287 (48.9%)
Away side in second leg through: 300 (51.1%)
Atlético Madrid v Bayer Leverkusen (4-2)
Home side in second leg through: 28 (96.6%)
Away side in second leg through: 1 (3.4%)
• The only side to fail to successfully defend a 4-2 lead at home were Barcelona, after a 4-1 loss against Metz in the 1984/85 European Cup Winners' Cup first round.
Monaco 3-5 Manchester City
Home side in second leg through: 4 (40%)
Away side in second leg through: 6 (60%)
Bonus UEFA Champions League round of 16 stat ...
How many seeds (home in second leg) have gone through
7: 2005/06, 2010/11
6: 2006/07, 2011/12, 2012/13, 2014/15, 2015/16
5: 2003/04, 2004/05, 2008/09