Away goals rule: Should Uefa scrap a system that may no longer serve its purpose?
A goal is a goal is a goal is a goal, right? Well, yes, apart from when it's an away goal in a drawn two-legged European tie.
Introduced by Uefa in the 1960s, the away goals rule provides a winner in ties that end level by assigning greater value to goals scored by visiting sides. It has become such a staple of football that commentators regularly utter cliches about away strikes being "all-important" and "counting double".
However, to its chief detractors - a group principally made up of Europe's top coaches - not only has the rule been left behind by football, it now actively works against the attacking ideals it hoped to promote.
If they have their way, the rule will be ditched in favour of something they feel is more in keeping with the modern game.
With the Champions League and Europa League group stages getting under way this week, BBC Sport digs through the stats to see what impact away goals could have on teams later in the competitions and whether the rule is indeed now past its sell-by date.
Back in the day...
The away goals rule was introduced in the now defunct European Cup Winners' Cup in 1965, making its debut in the European Cup two years later, initially in the first round before being extended to the second the following season and the whole tournament from the 1970-71 season.
It was principally devised to avoid the need for costly third games and to end the need for a coin toss should this game also end all-square. (Penalty shootouts were yet to be widely adopted.)
This was not the only reason. Back in the 60s, travelling around Europe was neither quick nor cheap and players were often heading into the unknown to face opponents of whom they had little or no prior knowledge, on pitches of wildly varying quality.
It is hardly surprising then that away wins in two-legged ties were so rare and visiting teams so eager to dig in for 90 minutes in order to claim a score they could overturn at home - a tactical approach Uefa were keen to discourage.
Only 18.98% of legs in European Cup ties were won by the away team prior to the introduction of away goals.
In theory, the rule dangled the carrot of a more valuable away goal, encouraging greater attacking intent. A 3-1 away defeat was now preferable to losing 2-0.
And for the past 51 seasons, the rule has remained as the principal means of separating sides level after two legs.
Three classic ties decided on away goals
Roma's Champions League quarter-final with Barcelona last season looked over when the Spanish side won the first leg 4-1.
But when Kostas Manolas headed in a late goal in the second leg, Edin Dzeko's away goal a week earlier proved crucial and saw Roma through with a 3-0 win on the night, avoiding the need for extra time and penalties.
Manchester City fell foul of the away goals rule in manager Pep Guardiola's first season in charge.
City went out in the last-16 stage after a thrilling 6-6 draw with Monaco over two legs.
They had won the first leg 5-3 but despite grabbing an away goal of their own in the second leg were knocked out because of the three goals conceded in Manchester.
One of the most dramatic Champions League ties in recent memory came at Stamford Bridge in 2009.
After drawing their first leg against Barcelona 0-0 at the Nou Camp, Chelsea were heading into a second successive Champions League final thanks to Michael Essien's first-half strike.
But, deep into stoppage time, midfielder Andres Iniesta scored a spectacular equaliser which sent Barcelona through at Chelsea's expense on away goals. The Spanish side went on to beat Manchester United in the final and win Guardiola a first European title.
This decade, 28.41% of games in the Champions League group stages onwards have been won by visiting teams - a significant increase on the 1960s.
"The coaches think that scoring goals away is not as difficult as it was in the past. They think the rule should be reviewed and that's what we will do," said Uefa deputy secretary-general Giorgio Marchetti after the annual gathering of Europe's top coaches in Nyon, Switzerland.
There is a logic behind the coaches' argument - pitches are better now, travel more luxurious and media and scouting so advanced that teams are fully prepared for what awaits them in even the farthest-flung parts of Europe.
The stats do indeed show that home sides are keeping fewer clean sheets in two-legged European ties than in previous decades...
The rule's critics argue that things have now tipped too far in the away side's favour, forcing home teams to adopt increasingly defensive tactics out of fear of conceding.
Former Arsenal boss Arsene Wenger has been banging this drum for a decade. "I believe the tactical weight of the away goal has become too important," he told a conference in 2008.
"Teams get a 0-0 draw at home and they're happy. Instead of having a positive effect it has been pushed too far tactically in the modern game. It has the opposite effect than it was supposed to have. It favours defending well when you play at home."
Teams are certainly scoring less at home now than they have in any previous decade of European club competition...
One additional factor to consider is extra time, which is played when teams also have the same number of away goals. However, the away goals rule remains in force, meaning the away side get 30 minutes more than their opponent in which to capitalise.
So what is the solution?
In the past, Wenger has suggested that away goals should only come into force beyond the 90 minutes of the second leg, in extra time - which is how the rule applies in the League Cup in England.
Others feel that abandoning away goals and going straight to a penalty shootout if scores are level after 180 minutes is the way to go.
And there are those who believe away goals add to the excitement of a tie and that the rule should stay.
Uefa will decide on the matter in due course. In the meantime, you can have your say below...
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