CHAPEL HILL, N.C. — North Carolina coach Roy Williams has made it clear to his players for years.

When that ball goes up, the Tar Heels had better put a body on someone to box out or work to get inside position and tap a missed shot back up on the glass. And those wing players, well, they need to run in and chase down rebounds or risk earning a seat alongside their Hall of Fame coach.

That relentless approach made the Tar Heels the nation’s best rebounding team and helped them earn a No. 1 seed for the NCAA Tournament’s South Region. It’s a key reason why they’re still alive, too, after surviving a scare to reach the Sweet 16 and face Butler on Friday in Memphis, Tennessee.

“It’s part of our job,” junior swingman Theo Pinson said. “It’s what we’re taught to do, and we need to do it.”

And no team has been this good at it in years.

North Carolina (29-7) leads the country in rebounding margin at plus-13.1, the highest average by a Division I team since Michigan State outrebounded opponents by more than 15 a game during a Final Four season in 2001. It touches everything the Tar Heels do, from snatching down a miss to secure a defensive stop, to the outlet pass that kickstarts their fast-paced offense and ultimately the work on the offensive boards that gives them extra chances around the rim.

“You’ve heard me say that a prospect came in one time and said, `I didn’t realize there was an 11th commandment: Thou shalt box out,”’ Williams said in January. “But that’s the way I treat the game.”

This year’s Tar Heels lack the kind of dominant rebounder they had in Associated Press All-American Brice Johnson, who averaged 17.0 points and 10.4 rebounds during the Tar Heels’ run to last year’s NCAA championship game.

Six-foot-10 senior Kennedy Meeks leads UNC at 9.1 rebounds per game, but UNC gets plenty of production through its lineup; 6-9 senior Isaiah Hicks (5.7) and 6-11 freshman Tony Bradley Jr. (5.3) work inside, while the team’s perimeter length shows up with the 6-6 Pinson (4.4) and 6-8 Justin Jackson (4.7).

“I told them, gave them (Louisville coach) Rick Pitino’s old line about `Those guys aren’t walking back in that door,”’ Williams said of losing Johnson and four-year starter Marcus Paige.

“So we started emphasizing it the first day: we’ve got to get rebounding from five people on the defensive boards. You hear us on the bench if sit close enough, all the time we’re screaming: `You’ve got to get to the boards.”’

UNC’s work on the offensive glass stands out in particular, with Pitino noting in February that it was “an offense unto itself.”

Consider: the Tar Heels lead the country in KenPom.com’s offensive rebounding percentage — a measure of possible offensive rebounds collected — at 42.2 percent. And that means the Tar Heels get roughly seven more shots per game than their foes.