THE match that defines a season. Regardless of finishing positions in the championship, the stakes are incredibly high when England take on Ireland in Twickenham this evening (5.0) with the progress of both coaches to be quantified by the result.
If England triumph, it will round off a superb championship for interim coach Stuart Lancaster, who has revitalised the national side, on and off the pitch, following their depth-plumbing World Cup experience.
It would also make it extremely hard for the English RFU not to give the job to Lancaster on a full-time basis -- although there are no guarantees as the 'up through the ranks man' is always susceptible to sacrifice on the southern hemisphere altar (as Ulster proved with Brian McLaughlin).
Even if England are beaten, last weekend's victory in Paris means they can still reflect on a decent championship with a revamped squad, though Lancaster's candidacy would be weakened.
Perception is everything and while Declan Kidney's job may not be on the line, the differences between victory and defeat carry huge significance for the Ireland coach also.
Win and it will have been a very good championship for Ireland, given the persistent challenges thrown in their way.
Overcoming the loss of two Lions captains, their forwards coach, other injury and unavailability complications, refereeing inconsistency and a fixture list of four matches on successive weekends by winning three matches and drawing in Paris, would be a considerable achievement. It would also add to the sense of what-might-have-been surrounding the opening loss to the Welsh.
However, were Ireland to lose today, the complexion would alter entirely. Then it would become a championship that only yielded victories over Italy and Scotland, the wooden spoon battlers, at home together with squandered opportunities against Wales, France and the English.
It would also throw Kidney's selection policy into a different perspective. Opting to keep faith with the same group that, ultimately, failed at the World Cup, only for them to come up short again, would provide Kidney's critics with easy ammunition.
A tough station, with incredibly tight margins, and it is difficult not to empathise with the Ireland coach. He also has the rankings-dependent World Cup seedings to negotiate, which placed an emphasis on experience and results ahead of long-term development.
That provides further incentive today for if Ireland win away to the fourth-ranked team in the world, they will climb to fifth. Lose and they slip back to eighth -- especially precarious with three Tests in New Zealand to follow in the summer.
It all adds to a fascinating contest which promises to go down to the final few minutes, particularly with Nigel Owens on the whistle, and as the week has progressed the sense that England are ready to burst Ireland's Paddy's Day bubble has grown increasingly powerful.
Wall of fire
Kidney has characterised his career with an ability to extract motivation from opponents' media offerings in the build-up to big matches. There is no better coach than Ireland's at finding material to pin up on the dressing-room wall and last year it worked a treat as England's over-confidence was demolished at Lansdowne Road, along with their Grand Slam dreams.
It is a lot different under Lancaster 12 months on. This time it is the Irish lobbing firebombs with great exposure given this week to Stephen Ferris' assertion that England are "bad losers", Rob Kearney claiming that "Ireland are better" and Donncha O'Callaghan saying the England jersey is "a red rag".
In each case, the Irish players were offering measured answers to provoking questions, but it still provided the sought-after headlines which were then offered to the English for their response.
Lancaster and his men did not rise to the bait and, even after their superb win in Paris, have been steadfast in their refusal to talk themselves up, as has happened so often in the past.
In the professional, scientific era of modern rugby, this pre-match stuff and guff should not matter, but history proves that it does -- particularly for the Irish who always perform best when they have a slight to put right or egos to shatter.
They have precious little to seize on against this unassuming England side, whereas Lancaster's men do, and you suspect handing the English these motivational spurs was never part of Kidney's plan.
England are a side that has built its progress this championship on a collective sense of purpose -- no showboaters or Flash Harrys, just honesty and industry. However, in their giant centre Manu Tuilagi, they have an individual that can scupper Ireland's ambitions if they do not close him down.
After being blown aside by Tuilagi in the World Cup warm-up defeat last August, Earls has extra incentive today and Ireland showed how effective their blitz defence can be against France and on the occasions they employed it against Scotland.
It looks the best way to go again, as Tuilagi may not be the best footballing centre in the world, but he is a beast on the charge and standing off him could be disastrous.
Owen Farrell may only be just out of his teens, but he has been one of the stars of the championship.
While Jonathan Sexton undoubtedly has the better all-round game, Farrell is improving with every outing and his greatest threat is his ability to kick penalty goals from all parts of the pitch.
It brings back memories of Jonny Wilkinson a decade ago when England could assume they would get points every time their out-half lined up a kick.
Ireland have had their issues with penalties in this Six Nations and if they stray into double figures again, Farrell will make them suffer.
Stop the pop
Ireland captain and hooker Rory Best was extremely diplomatic this week when asked about the scrummaging style of Dylan Hartley.
The Northampton hooker likes to stand up in the scrum and drive the opposition upwards, looking to 'pop' them, rather than set straight and square.
If allowed to get away with it, this is incredibly hard to counter and Ireland need Owens to keep tabs on exactly what is taking place at scrum time -- if he does, then Ireland have the front-row to wrest supremacy, as they showed at Lansdowne Road last year.
Tired but trusted
It has been a long, sapping campaign and Ireland stagger into Twickenham seeking to summon up the energy for one last push.
The postponement in Paris took away the opportunity for a mid-campaign breather and Kidney has done remarkably well to keep morale high after eight weeks in camp, using the likes of Ryanair raconteur Michael O'Leary and balladeer Christy Moore to lift spirits.
England have had the six-day turnaround this week, which helps the Irish and Kidney has tailored training to reduce the risk of burnout, but it will still require a mighty effort to overcome the English in front of their own fans.
The statistics are positive -- only one defeat in the last eight Six Nations meetings -- but England bring tremendous pride and belief into this encounter.
There is unlikely to be more than one score in it and do not be surprised if Ronan O'Gara has a part to play in the result.
O'Gara has performed well off the bench, but has yet to prove a match-winner and grab the headlines in this Six Nations -- the first time that has happened since he broke through in 2000.
Verdict: Ireland, barely