Individually, they have had their days in the sun - the Ospreys winning what began as the Celtic League four times, the Scarlets once and Cardiff Blues beating Toulon down in Marseilles to win the Amlin Cup five years ago.
Collectively, they had never done anything to compare with last weekend. Between them the four Welsh teams saw off Irish opposition in Cardiff, Swansea, Llanelli and Newport to produce figures which have not been seen on any international scoreboard for at least a quarter of a century:
Wales 4, Ireland 0.
It last happened back in the Eighties shortly before the try appreciated in value from four points to five. The wins could hardly come at a more relevant time, not that anybody would pretend for one moment that the regional Grand Slam will have any direct bearing on Ireland's pursuit of the ultimate Slam at the Millennium Stadium on Saturday.
Even allowing for home advantage in every case, the clean sweep by the Blues, Ospreys, Scarlets and Dragons took some doing against provincial opponents drawn from the top six of the GUINNESS PRO12. Appropriately, the Welsh roll began in extraordinary circumstances at BT Sport Cardiff Arms Park on Friday night.
Connacht negotiated their way into stoppage time in defence of a six-point lead which would have hoisted them to within two points of the play-off zone but for the Blues' prolonged finale. They went through so many phases that they were heading for close to three figures before Connacht finally cracked.
It took the Blues six minutes and 42 seconds to find a way through. By the time Joaquin Tuculet scored surely the latest try ever seen in the PRO12, Connacht had made countless tackles, conceded two penalties and lost one player - second row Mick Kearney to the bin.
Tuculet, aware that a try in itself would not be enough, had the presence of mind to touch down as far from the corner flag as possible. The clock had passed 88 minutes by the time Rhys Patchell held his nerve to land the decisive conversion.
The result caused a domino effect which accounted for Ireland's three title contenders within the next 48 hours. A solitary losing bonus point was all Munster, Leinster and Ulster had to show between them for their efforts.
Munster's four-match winning streak came to grief at the Liberty Stadium where Dan Baker's try and seven Sam Davies goals (including one drop goal) preserved the Ospreys' winning home run in Swansea. It also put them right back on track for the play-offs.
Confirmation of their return to the top four came later in the day thanks to the Scarlets keeping their part of the Welsh bargain. Not one bit pleased at allowing first Ulster and then Munster to steal away from Llanelli with improbable draws, Wayne Pivac's team made no mistake against Leinster.
All-square down the road in Swansea the previous week, the former treble champions of Europe finished up some way short of a losing bonus point never mind a second successive draw. Rory Pitman's third try 12 minutes from time removed any danger of the Scarlets suffering from another late dose of generosity.
They are now just three points behind Connacht and one better off than Edinburgh in what would appear to be a three-cornered race for a top-six finish and qualification for the European Champions' Cup.
The Scarlets' five remaining fixtures are all against teams beneath in the lower half of the table, starting, crucially, with Edinburgh at home. The other four matches are against Zebre, Dragons (both away), Blues (home) and, finally, back to Italy and Treviso.
As the Scarlets absorbed the significance of their Leinster victory, the Dragons found themselves in the challenging position of completing the cleanest of Welsh sweeps with the toughest ask of all.
Ulster, joint top at the start of the weekend, arrived at Rodney Parade on Sunday with no shortage of internationals despite losing two of their Irish contingent - Roger Wilson and Luke Marshall - to suspension.
Darren Cave, at 27 a youthful veteran of more than 150 Ulster matches, took over the captaincy, the dazzling Craig Gilroy scored two tries from full back, made a third for 20-year-old Ross Adair on debut and they still came up short against super-charged opponents.
Only the Dragons could lose to Zebre one week and show Ulster a clean pair of heels the next. The cleanest pair of all belonged to Carl Meyer, a 23-year-old South African whose response to being promoted from Ebbw Vale in the Welsh Premiership was to touch down the Dragons' decisive third try six minutes from time.
The move would have been worthy of winning any game, Hallam Amos making it possible with a hand-off followed by an exquisite one-handed pass tight on the touchline for the supporting Meyer to apply his gymnastic finish.
Nowhere did Welsh popularity soar higher than in Glasgow. After securing a maximum-point home win over Zebre, the Warriors weren't the chief beneficiaries of the Welsh quadruple, they were the only ones.
All three of their major rivals for the title had lost ground and suddenly last year's Finalists found themselves four points clear at the top eight days after being knocked off their perch by Munster. The leaders are one of only two contenders with a 100 per cent home record in this season's PRO12.
Home records of the Guinness PRO12 clubs:
P W D L To play at home:
Warriors 9 9 0 0 Blues, Ulster.
Ulster 8 8 0 0 Blues, Leinster, Munster.
Ospreys 9 8 1 0 Zebre, Warriors.
Scarlets 9 7 2 0 Edinburgh, Blues.
Leinster 9 7 0 2 Warriors, Treviso.
Connacht 8 6 1 1 Ulster, Warriors, Ospreys.
Munster 8 6 0 2 Connacht, Treviso, Dragons.
Edinburgh 8 5 1 2 Munster, Zebre, Leinster.
Dragons 8 4 0 4 Leinster, Scarlets, Edinburgh.
Blues 9 4 0 5 Ospreys, Zebre.
Zebre 9 3 0 6 Scarlets, Connacht.
Treviso 8 2 1 5 Dragons, Ospreys, Scarlets.
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