At the end of the group phase, all third-placed teams will be ranked on the number of points they've won in their respective group.
Teams with the same number of points will then be separated on goal difference, goals scored, their fair play record at the competition and, finally, if none of those factors sets them apart, their UEFA national team coefficient ranking.
Here's how the teams currently in third rank:
What does this mean for Northern Ireland?
Northern Ireland are third, can't be overtaken by Ukraine, currently have three points and a positive goal difference. The bad news is they play world champions Germany next. Given the remaining fixtures, Northern Ireland's points total - and place in the knockout round - is under threat if they don't get a result in their final fixture.
What does this mean for Republic of Ireland?
The Republic of Ireland's mission is clear: beat Italy. With just one point from their first two games, a draw with the Azzurri would not be enough. That's because in Group A Albania have finished with three points and in Group D the bottom two teams play each other, with one of those sides inevitably ending up with a better record than Ireland, should they draw with Italy.
What does this mean for England and Wales?
England, currently on four points, can still finish third but would have to sustain a heavy defeat to Slovakia to miss out on a best third-place spot. Wales, though, like Northern Ireland, will be eager to strengthen their position of three points with a neutral goal difference.
However, both England and Wales can also finish top of Group B - and the winner of that pool plays the best third-place team from Group A, C or D.
The opponent will be determined on the final third-place rankings - so, should England or Wales win Group B, they won't know who they'll face in their last-16 tie (which takes place on Saturday in Paris) until the final group games have been played on Wednesday night.
Below is a rather complicated-looking table which shows the different permutations. It's worth noting that should Northern Ireland beat Germany and Poland get a result against Ukraine, England or Wales could play the world champions in the first knockout round...




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