Then there was four...
After 132 games over 22 rounds just four of the 12 participating teams now go forward to the knock-out phase in the
quest to find the Pro12 champions of 2012/13.
All bar one of them, Scarlets, have competed in the play-offs before, although the Llanelli-based team did get their name on the trophy in 2004, the first season the tournament was played on a home and away basis.
In the three seasons since the introduction of the play-offs, the side that has finished top of the table has gone on to win the title just once, Munster in 2010/11.
On the other two occasions Leinster have not been able to capitalise on the home advantage granted to them for finishing top of the pile, losing out twice to Ospreys in finals played at the RDS.
Top seeds Ulster spent 15 of the 22 rounds leading the table, and having slipped down to third during March, bounced back up to lead for the final three rounds.
Their opponents, Scarlets, despite losing to Benetton Treviso last time out, were still able to secure the last play-off spot as their nearest rivals, Ospreys, also lost in their final game against Leinster in Dublin.
It is appropriate that the other two play-off participants, Leinster and Glasgow Warriors, face each other as they had both been battling for home advantage over the second half of the season, each of them taking over the top spot in turn when Ulster faced a dip in form.
In the end Leinster took advantage of a better run-in and held on to the second place they had claimed three rounds out.
And so the scene was set, the last four teams know just 80 minutes stands between them and a place in the Pro12 final on May 25.
We look at the two semifinals!
Friday, May 10
Ulster v Scarlets
(Ravenhill - Kick-off: 19.45; 18.45 GMT)
The Scarlets travel to Ulster on Friday for their Pro12 play-off with their minds firmly fixed on stepping up for their very best performance this season.
The Scarlets may go into their first encounter of the knockout phases of the Pro12 as underdogs against an impressive and award winning Ulster side this season; but the challenge of overcoming the league leaders at home is a position the travelling Scarlets team will relish.
After 22 rounds of the competition, the Scarlets have achieved their target set out at the start of the season of being one of the four teams from twelve to go forward to the knockout phase in the quest to find the Pro12 champions of 2012/13.
The Scarlets will take confidence from a series of big away wins over the course of the league season including results on the road in Glasgow, Munster, Edinburgh and Connacht.
The Scarlets expect a big battle knowing that top seeds Ulster spent 15 of the 22 rounds leading the table, and having slipped down to third during March, bounced back up to lead for the final three rounds.
The Ulstermen have lost just one of their last six encounters in the competition against Edinburgh in late March 8-14. Their only defeat to a Welsh region was against the Ospreys in February 12-16.
The Scarlets had won five games in a row in the competition before their defeat to Treviso last Friday. Ulster have won five of their last six contests against the Scarlets - the West Wales region's last victory over the Irish province came in October 2011 at Parc y Scarlets 24-17.
Simon Easterby said: "All that happened last week against Treviso has been firmly put to bed now. We are one of four teams that have 80 minutes of rugby in front of them to prove their ability and battle for a place in the final.
"This is where we wanted to be this season, we've achieved a top four finish, now we have to bring everything we have to this game to back up all the hard work we've put in throughout the season."
Simon Easterby opts for just three changes to his match day squad from last weekend against Treviso; with Wales international outside half Rhys Priestland returning to the starting line-up after two appearances from the bench on his return from injury.
Hooker Emyr Phillips replaces Matthew Rees on the bench (groin injury) and Gareth Owen comes onto the bench for Gareth Maule (thumb injury). Back row Rob McCusker who was withdrawn late against Treviso with a rib injury returns to captain his side.
The last time West Wales rugby held the trophy was in the Celtic League's first season in 2004 however history lessons and memory lane have not formed part of the preparations this week from Head Coach Simon Easterby who lifted that trophy as the region's captain.
He said: "Most of our players would have been young teenagers at that time, so I'm not sure that those great games are really very relevant. What we have looked at this week are games were the unexpected has happened; we'll take strength from some of the big away wins we've achieved this season at tough grounds like at Munster, Glasgow and Connacht.
"We have played some good rugby to get ourselves to this position and some strong winning rugby at places we were not expected to win at and that's our focus going into this game. We've used last Friday as a motivator to produce the sort of performance you'd expect from a top four side and from the Scarlets jersey.
"We have plenty to face in coming up against Ulster. They have been pretty much unbeatable this season and they are in the driving seat at home.
"What we have to bring is our commitment, ambition and desire to win. That has to be in body as well as mind and shown in our work rate and intensity on the field.
"Sides will be threatened by the players we put on the park, there's a good mix in our team and we know the quality that key Scarlets players can bring to our performance.
"We'll have a good travelling support out there who will get behind us and make their voices heard. We want to give them a performance they can really cheer about after the disappointment of our final home game.
"It's going to be a huge night for us as Scarlets - we have this opportunity in front of us now and we'll give it everything we have."
Ulster finished on top of the pile in the Pro12 table this season to qualify for a first play-off since 2010/11.
The Ulstermen have lost just one of their last six encounters in the competition; 8-14 at Edinburgh on 22nd March. Ulster’s solo
defeat to a Welsh region in their last nine such matches was 12-16 at home to Ospreys on 8th February.
Scarlets five game winning run in the PRO12 came to a shuddering halt with their 17-41 reversal at the hands of Benetton Treviso at Parc y Scarlets in round 22. The Welshmen’s last five fixtures against Irish provinces have all been won by the home side on the day. This is Scarlets first appearance in the play-offs.
Ulster have won five of their last six contests against Scarlets with the Welshmen’s only success during that time being 24-17 at Parc y Scarlets on 29 October 2011. Scarlets have not been victorious at Ravenhill since the opening round of the 2008/09 season.
Ulster: 15 Jared Payne, 14 Andrew Trimble, 13 Darren Cave, 12 Stuart Olding, 11 Tommy Bowe, 10 Paddy Jackson, 9 Ruan Pienaar, 8 Nick Williams, 7 Chris Henry, 6 Robbie Diack, 5 Dan Tuohy, 4 Johann Muller (captain), 3 Declan Fitzpatrick, 2 Rory Best, 1 Tom Court.
Replacements: 16 Rob Herring, 17 Callum Black, 18 Ricky Lutton, 19 Iain Henderson, 20 Mike McComish, 21 Paul Marshall, 22 Michael Allen, 23 Peter Nelson.
Scarlets: 15 Liam Williams, 14 George North, 13 Jonathan Davies, 12 Scott Williams, 11 Andy Fenby, 10 Rhys Priestland, 9 Aled Davies, 8 Rob McCusker (captain), 7 Josh Turnbull, 6 Aaron Shingler, 5 Johan Snyman, 4 George Earle, 3 Samson Lee, 2 Ken Owens, 1 Phil John.
Replacements: 16 Emyr Phillips, 17 Rhodri Jones, 18 Jacobie Adriaanse, 19 Jake Ball, 20 Sione Timani, 21 Gareth Davies, 22 Owen Williams, 23 Gareth Owen.
Referee: Alain Rolland (Ireland)
Assistant Referees: Andrew McMenemy, Adrian Graves (both Scotland)
TMO: Jim Yuille (Scotland)
Saturday, May 11
Leinster v Glasgow Warriors
(RDS, Dublin - Kick-off: 19.35; 18.35 GMT)
Leinster front row Cian Healy is expecting a bruising tussle against his Glasgow Warriors counterparts when the sides meet in Saturday's Pro12 semifinal.
In a repeat of last year's semifinal, Leinster welcome the Warriors to the RDS, having edged home 19-15 last season despite being outscored by two tries to one with both Dougie Hall and Stuart Hogg going over.
This season, the Warriors are the Pro12 top try-scorers while Leinster have also impressed of late and ran in four tries against defending champions the Ospreys last time out.
But prop Healy, who has been selected for the British & Irish Lions tour to Australia this summer and was one of the try-scorers last time out, believes the battle up front will be just as important.
"They're bruisers in the pack. Defensively they're strong and their record stands up," Healy told the Irish Independent.
"It's a strong scrum, but it's nothing we haven't dealt with before. You have to back yourself and that's what we'll do.
"Every team can be broken down and we'll be prepared for Saturday."
This is Leinster’s fourth successive home play-off game at the RDS, all of which they have won.
In fact the Leinstermen have won their last six semifinal fixtures in all competitions. Leinster’s only loss in the Pro12 since round 11 was 18-22 at home to Ulster on 30th March.
Leinster are unbeaten in their last ten encounters against Scottish opponents in all tournaments since Glasgow Warriors won 23-19 at the RDS in the PRO12 on 17th September 2011.
This is Glasgow Warriors second successive appearance in the play-offs, and a repeat of last year’s fixture at this stage when they also faced Leinster at the RDS, but went down 15-19.
The Warriors have won three of their last four contests with Irish provinces.
Leinster are unbeaten in their last six clashes with the Warriors in all competitions whilst Glasgow have been victorious only once in nine previous visits to the RDS.
Leinster: 15 Rob Kearney, 14 Fergus McFadden, 13 Brian O'Driscoll, 12 Gordon D'Arcy, 11 Isa Nacewa, 10 Johnny Sexton, 9 Isaac Boss, 8 Jamie Heaslip, 7 Sean O'Brien/Shane Jennings, 6 Kevin McLaughlin, 5 Devin Toner, 4 Leo Cullen (captain), 3 Mike Ross, 2 Richardt Strauss, 1 Cian Healy.
Replacements: 16 Sean Cronin, 17 Jack McGrath, 18 Jamie Hagan, 19 Quinn Roux, 20 Shane Jennings/Rhys Ruddock, 21 John Cooney, 22 Ian Madigan, 23 Andrew Conway.
Glasgow: 15 Stuart Hogg, 14 Sean Maitland, 13 Sean Lamont, 12 Alex Dunbar, 11 DTH Van der Merwe, 10 Peter Horne, 9 Niko Matawalu, 8 Ryan Wilson (captain), 7 John Barclay, 6 Josh Strauss, 5 Al Kellock, 4 Tim Swinson, 3 Jon Welsh, 2 Pat MacArthur, 1 Ryan Grant.
Replacements: 16 Fraser Brown, 17 Moray Low, 18 Ed Kalman, 19 Tom Ryder, 20 Rob Harley, 21 Henry Pyrgos, 22 Ruaridh Jackson, 23 Mark Bennett
Referee: Pascal Gauzere (France)
Assistant Referees: Marius Mitrea, Giuseppe Vivarini (Italy)
TMO: Carlo Damasco (Italy)
* Pro12 Final - Saturday, May 25 (kick-off 16.45; 15.45 GMT)
In the event of a draw:
* In the event of a tie at full time in a Pro12 semifinal or the Final then extra-time will be played, consisting of two periods, each of 10 minutes.
* If the score is still tied after extra-time, the team that has scored the most tries in the match will be deemed the winner.
* If the teams are still tied on points and tries at the end of extra-time then the winner will be determined by a place-kick competition. In the event that a place-kick competition is required, each team will nominate three goal kickers who will take six place kicks between them from different positions along the 22-metre and 10-metre lines. The Team with the most place kicks scored will be the winner.
* If the Teams are tied at the end of their six kicks, then sudden death will apply. Only players in the playing area at the final whistle of extra-time may be nominated for the place-kick competition. No substituted players or players who have been shown a red card may take part at any time including any player who has received a yellow card and who remains in the sin bin at the time of the final whistle of extra-time.