Behind Clemson's ascendance to the top of the rankings and the national conversation over the last few years has been the play of its defense, coached by Brent Venables since he left Oklahoma after the 2011 season.

The S&P+ rankings go as far back as the 2005 season. In those 10 seasons. Venables has coached seven defenses that ranked in the top 10. The others ranked 17th, 34th and 12th.

Venables' 2014 Clemson defense was perhaps his shining achievement. The Tigers finished second nationally in S&P+ and sent four players to the NFL, including first round selections Vic Beasley (DE) and Stephone Anthony (LB).

After Clemson lost five defensive starters, nearly all of them foundational pieces, the assumption was the Tigers would fall a good ways in 2015 and be forced to rely on playing solid but not elite defense. But now the defense ranks fourth in S&P+ without a particularly imposing opponent left on the regular season schedule.

With more than a decade of top defenses in Norman and Clemson, it may be time to ask whether Venables is the best defensive coordinator in college football.

Venables vs. the run

At the most basic level, the Venables defensive philosophy is pretty standard. He wants to stop the run on early downs, take away the opponents' best receiver, and attack with zone blitzes on third down. What makes his defenses so effective are the ways in which he is able to adapt to his personnel to accomplish these goals.

Let's start with his approach to stopping the run on standard downs. There are two dominant schools of run defense in college football. First, there's the inside-out school that looks to spill the ball outside of the tackles and run it down in team pursuit. Cover 4-based defenses like Michigan State and TCU are proponents of this strategy, filling interior gaps quickly, so runners are bounced outside to the safeties.

The other school is to play the run outside-in, with force players operating at the first level of defense rather than the seconds. This is more common with 3-4 teams, but it's also how Clemson plays. In 2014, the Tigers loved to accomplish this either with 4-3 Under fronts that played a Sam linebacker outside in a 9-tech to keep the ball focused inside ...

... or occasionally with fire zone blitzes that created 5-2 under-style fronts and allowed Beasley to get one-on-one matchups.

The Clemson defensive backs each have straightforward assignments, typically keying off the skill player they'll have in coverage and rushing to the action when they get a run read.

With the ball funneled inside at the line of scrimmage by first level contain players, the Clemson safeties are then instrumental in bringing down the ball carrier and acting extra men in the box. In 2014, the second and third leading tacklers on the team were safeties Robert Smith and Jayron Kearse. In 2015, free safety T.J. Green tends to play that role while inside linebackers B.J. Goodson and Ben Boulware are first and second.

The 2015 strategy for Clemson is a bit different, as the Tigers tend to play fewer Under fronts and accomplish the same goal from the 4-3 Over with a 9-tech defensive end. As you can see from the clip below, the Clemson DEs are looking to set a hard edge.

With 6'5, 275-pound Kevin Dodd and 6'3, 270-pound Shaq Lawson, the Tigers benefit more from using ends to set the edge rather than using linebackers or DBs, who can now patrol the middle of the field and fire downhill when they read run.

Clemson's eight-man fronts surging downhill makes it very difficult for blockers to get effective double teams or reach linebackers who are not hesitating to attack the line of scrimmage. If the offensive line maintains a double team for too long, the linebackers will fly through unimpeded to the running back. If the line comes off the double team to block the linebackers, Clemson's very strong defensive line is now threatening to make the tackle behind the line of scrimmage.

The only hope is to blow open a hole quickly and have a tailback with enough vision and explosiveness to make the most of it, or to fool Clemson's second level defenders and catch them in the wash:

Venables vs. the pass

It's all fine and well for teams to gang up to stop the run, but how do you avoid getting shredded by the opposing team's passing game?

As you can tell from how he's schemed his teams differently to stop the run, Venables has extensive knowledge of multiple defensive schemes that he'll employ based on his personnel in a given year. His first priority is to stop the run so he can set up his defense to defend third and long with the 30 front (three down linemen), a pass-rushing package from which they can bring a wide variety of different blitzes:

Since there are only three down linemen, it's hard for the offense to tell which players will rush. The confusion is increased by the linebackers and safeties moving all over the formation and threatening blitzes from different angles before the snap.

The other major part of Venables's philosophy is to always take away the opposing team's best receiver. In 2014 Clemson made heavy use of cover 6, a coverage with the boundary corner and safety bracketing the receiver to that side of the field. Many passing teams love to isolate their best receiver by simply loading up the field side and then working against the solo boundary receiver, but Venables would cancel that out with Garry Peters playing press coverage with a safety over the top.

While they'll still bracket the boundary at times, in 2015 the Tigers have a much simpler solution for erasing opposing teams' top receiver: Mackensie Alexander.

Against Notre Dame, Alexander tracked Irish star WR Will Fuller and held him to 37 yards on two catches. Against the Seminoles, Clemson moved Alexander around to take away Kermit Whitfield, who managed only 21 yards on three catches.

Most passing attacks are going to get throttled if a single defender can take away a receiver to that extent.

The rest of the defense plays mostly pattern-matching coverages with the aim of taking away the deeper throws before rallying to the shorter throws. Whereas many cover 3 teams will assign their linebackers to the running backs and leave it at that, Clemson will trade off receivers and rely on tackling rather than ensuring that receivers stay covered throughout their routes.

This leaves the Tigers vulnerable to teams with great timing and teams who can get good matchups on their defenders. But Clemson tends to match up fine on the best wide receivers and will disrupt timing with the pass rush and occasionally jams at the line of scrimmage.

The Tigers are currently on a trajectory that likely pits them against North Carolina in the ACC championship game before entering the playoffs against a much less clear field. Their ability to load up to stop the run while erasing opposing teams' best WR with Mackensie Alexander is going to play well in a season with very few elite passing attacks amongst the elite teams. If Clemson's success continues, perhaps Venables will start to get more attention as one of the best defensive coaches in the country.