Two pillars of strength, a pair who fortify their goal-line with red-and-white wall and a partnership that is now so intuitive that, whenever fielded together, it must give the players ahead of them on the pitch, the coaching staff and all the club's fans confidence.
Over 18 months ago, Mertesacker was criticised by the media. The towering German, an accomplished footballer with Werder Bremen and the German national football team, did not require the pace necessary to stay toe-to-toe with the division's fleet-footed attackers.
Koscielny, too, was not exempt from negative comments and, around the same time Per had to rise above it, Laurent also had to handle detractors who noted his questionable positional nous.
Since displacing captain Thomas Vermaelen, though, the two have never looked back and, despite the one concession versus Aston Villa on Monday, January 13, their combined performances underlined a key aspect of Arsenal's 2013-14 title charge - their great understanding.
Together, they won 53% of their aerial battles, completed four interceptions, made 29 clearances, won the ball 100% of the time they attempted a tackle and only misplaced one of their 82 pass attempts (for the record, it was one of Koscielny's).
Individually, throughout the season, they have both been impressively consistent (data noted as averages per game).
However, their best form is realised when they are both fielded together, as demonstrated by the impressive fact that, in the past two years, when they have both featured in the full 90 minutes, Arsenal have not once been beaten.
When the Mertielny/Kosacker partnership starts and finishes a game, there are, on average, 0.5 goals conceded per game, zero losses, eight draws and, to date, 19 victories with 16 clean sheets from a possible 27 matches.
image: © Ronnie Macdonald