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Who needs what?
Group B
Before we go an inch further, let's lay out exactly what each of the sides in Group B need to reach the last 16. The last thing we need tonight is any confusion.
England missed the chance to seal a last-16 spot with a game to spare as they were held to a scoreless draw by the United States.
It means that all four teams still have a chance of reaching the next round going into the final group games.
England are in the strongest position with four points from two games and Southgate's side will advance if they avoid a four-goal defeat by Wales on Tuesday.
A win would guarantee England top spot, but they can still draw and finish top unless Iran beat the United States, or the Americans win by at least four goals.
England will also finish top if Iran and the USA draw and they don't lose to Wales by four or more goals.
Wales still have a chance of reaching the last 16 despite losing to Iran on Friday. To qualify, Wales must beat England and hope for a draw between the United States and Iran.
If the USA-Iran match does not end in a draw, Wales must beat England by four goals to advance.
The United States must win to progress, while Iran will go through with a draw unless Wales beat England.
Clear?
All on the line
Wales v England (19:00 GMT)
When the draw for the World Cup in Qatar was made, the fixtures decided, we all probably knew this game would have something very important riding on it.
So it has come to pass.
Tonight is last chance saloon for one of the home nations, Wales' last-16 hopes hanging by the thinnest of threads.
England are almost there, but after bursting out the blocks they developed a bit of a limp in game two. They will want to stage a late, confident drive for the line.
If England avoid defeat they go through and Wales go out.
Even if Wales win they could be in for heartbreak.
Live Reporting
Phil Dawkes
All times stated are UK
Heart's on fire
Wales v England (19:00 GMT)
It feels like this needs an early injection of fire. Something to get us all up and at it, that reflects the battle about to take place.
How about this...
No pain. No pain.
(Warning: Third-party content may contain adverts)
Who needs what?
Group B
Before we go an inch further, let's lay out exactly what each of the sides in Group B need to reach the last 16. The last thing we need tonight is any confusion.
England missed the chance to seal a last-16 spot with a game to spare as they were held to a scoreless draw by the United States.
It means that all four teams still have a chance of reaching the next round going into the final group games.
England are in the strongest position with four points from two games and Southgate's side will advance if they avoid a four-goal defeat by Wales on Tuesday.
A win would guarantee England top spot, but they can still draw and finish top unless Iran beat the United States, or the Americans win by at least four goals.
England will also finish top if Iran and the USA draw and they don't lose to Wales by four or more goals.
Wales still have a chance of reaching the last 16 despite losing to Iran on Friday. To qualify, Wales must beat England and hope for a draw between the United States and Iran.
If the USA-Iran match does not end in a draw, Wales must beat England by four goals to advance.
The United States must win to progress, while Iran will go through with a draw unless Wales beat England.
Clear?
All on the line
Wales v England (19:00 GMT)
When the draw for the World Cup in Qatar was made, the fixtures decided, we all probably knew this game would have something very important riding on it.
So it has come to pass.
Tonight is last chance saloon for one of the home nations, Wales' last-16 hopes hanging by the thinnest of threads.
England are almost there, but after bursting out the blocks they developed a bit of a limp in game two. They will want to stage a late, confident drive for the line.
If England avoid defeat they go through and Wales go out.
Even if Wales win they could be in for heartbreak.
The World Cup takes no prisoners.
Battle of Britain
One team has to win...
The other dare not lose...
Something has to give.