IT IS a simple equation. Four into three won't go in any back row.
Part-time captain Jamie Heaslip and full-time wrecking ball Sean O'Brien are automatic choices for Leinster coach Joe Schmidt.
From there, the old warrior Shane Jennings, 31, and blindside battler Kevin McLaughlin, 28, scrap it out for the third and final piece in this part of the jigsaw.
This constantly changing permutation may not last much longer as Rhys Ruddock, 22, Dominic Ryan, 22, and Jordi Murphy, 21, work to bridge the gap between bit-players and regular starters.
Increasingly, McLaughlin has gained the upper-hand over Jennings as O'Brien has grown into the openside role, when blindside and even number eight have often been seen as a better fit for his explosive talents.
"In a squad like this you're almost always looking over your shoulder and you know that if you miss out then you could face a longer spell on the sidelines," said McLaughlin.
The dark spectre of injury can push a player back to the end of a long queue. The under-rated McLaughlin knows this better than anyone. A stinger to his shoulder in the warm-up to the Ulster match was the most recent injury to threaten his place at the front of the line.
"It's always a bit nerve-wracking watching games because you have a lot of nervous energy to expend," he said, in appreciation of the quality of player on his tail.
"The likes of Rhys (Ruddock) and Dominic Ryan have really stepped up in recent weeks and are making an impression, which is great to see.
"With Jenno' (Shane Jennings), Seanie (O'Brien) and Jamie (Heaslip) all back in the frame this week, it's starting to get a bit congested in the back row. But that's exactly what you want.
"The greater the competition for places, the more intense training is. The enthusiasm all of the lads bring can only be a positive," he said.
While The Scarlets (January 12) and Exeter Chiefs (January 19) wait in the short grass in Europe, Leinster will search for their PRO12 League rhythm in Scotland this evening.
"Edinburgh will pose a big challenge for us this week. We went through a phase for a number of years where we couldn't buy a win there," he emphasised.
"We won a high-scoring game last year and snuck a win by the skin of our teeth earlier this season, so we're expecting them to be gunning for us.
"We'll need to bring a high level of intensity and it'll be another tight and physical battle," he quipped, despite the fact that Edinburgh have a 33pc win percentage from the 12 rounds of the PRO12.
"Under Michael Bradley they have put together some really good performances and their run in Europe last year was an indication of the threat they can pose. From our point of view, January is a huge month.
"We're still hanging on in both competitions and it's all to play for, but we're coming into Europe at a slightly different angle than last year."
That angle could be best described as obtuse.