NFL joint practices, rare just a few years ago, have grown quickly in recent preseasons. Bill Belichick has been driving the phenomenon and Chip Kelly isn’t been far behind, with one or more every year since he came into the league. The San Francisco 49ers are set to hold joint practices with the Houston Texans on Friday, and then the Denver Broncos next week.

On Wednesday, Kelly told reporters:

“It’s a great way to gauge yourself [as a team]. I’ve done it in the past a couple times with the Patriots and once with the Ravens. We thought it was quality work in the three years that we did it. We’re going to do it twice this year, not only with the Texans on Friday and then play them on Sunday, but then next week when we go out to Denver.”

Niners defensive coordinator Jim O’Neil is pumped as well. Thursday, he said:

“I’m really excited about it, ... [F]or us defensively, it’s going to be huge just from a player evaluation standpoint, and then it’s a totally different offensive scheme. It’ll be a lot more two-back formations, stuff we need to see to get ready for the season. So, it’ll be a big test for our defense.”

This year around half of the teams will hold at least one joint practice, usually right before a preseason game. Five teams will hold two, and the Buccaneers have three. Only a few of the oldest-school coaches, such as Andy Reid and the recently fired Tom Coughlin, remain firmly opposed.

In many ways, a joint practice is better than a preseason game, not least because season ticket holders don’t have to pay for them. And coaches can focus on the most meaningful plays in 7 on 7s and 11 on 11s: red zone, 3rd and short, 3rd and long, whatever they want.

The best players barely see the field in preseason games — Philadelphia QB Sam Bradford got just three snaps against Tampa Bay Thursday night — but they participate fully in joint practices, barring injury.

While the league would like to replace two preseason games with two additional regular season games, many players and writers would rather just cut two of the four non-counting contests and jointly practice before the other two.

So what’s the downside? Besides fear of giving away your secrets — always a danger when Bill Belichick is hanging around — there is a real risk of fights breaking out.

Every training camp is full of guys with a lot at stake — the difference between the NFL’s minimum $435,000 salary and bragging rights for life, versus nothing and the collapse of your life-long dream. Playing against teammates takes a bit of the edge off, but making your case against strangers adds the angle of setting the tone against a possible future opponent. The 49ers used to hold joint practices on occasion with the Oakland Raiders, and things occasionally got a little frisky.

Coaches need to set a strong tone against fighting, or expect scuffles. Not surprisingly, the teams that have the most fights year after year, such as the Dallas Cowboys, are the ones that wink at or openly encourage that feisty attitude.

After his team had two brawls against the Raiders in their 2014 joint practice, Jerry Jones told reporters that he didn’t mind.

"Seriously, that passion, that's mostly what I'd hoped what we would get out of getting these teams together.”

Sure enough, Dallas had even bigger fights agains the Rams last year.

The Texans and Washington actually had to stop their combined practice last summer because the scrapping got out of hand. But, like the Cowboys, Washington is a team that proudly signs attitudinal guys (such as Chris Culliver).

Kelly takes the opposite approach. Wednesday, he told reporters that he didn’t plan to issue any special warnings against fighting Friday, but only because he’s been very clear from his first day with the team that fights are not OK.

“[The players] understand that you have to play with emotion, not let emotion play with you. It’s the same rules that you have in a game. If you get in a fight in a game, you’re not going to be around. So, it’s the same thing. We’re trying to get a good quality practice. We’re not trying to turn it into a WWE wrestling match.”

O’Neil was on the same page.

“I think coach [Kelly] and [Texans coach] Billy O[’Brien] will do a good job with explaining to the guys, ‘Hey, we’re not here to fight. If you’re going to fight, you’re going to be out of here.’ So, I’ll let them handle that. I’m not worried about that though.”

The key to a good joint practice is the same as the key to a regular practice: planning and organization, which are strengths for Chip. He clearly enjoys hanging out with other coaches, even when they compete, and has been known to spend his summers visiting current and retired coaches at every level.

This week’s work with Houston is no different.

“Yeah, we’ve spent a lot of time [laying out the practice]. I’ve known [Houston Texans head coach Bill O'Brien] Billy for a long time, so back and forth, what they want to get out of it, what we want to get out of it. Then our coordinators talk on each side of the ball, so ... It’s kind of a give and take on both sides, what everybody wants, ... and they’ve been great. It’s a great organization...”

When Kelly says he’s know O’Brien a long time, he’s not kidding. They’ve visited each other since the early 1990s when O’Brien was a position coach at Brown and Chip was an assistant at the University of New Hampshire. Kelly drove down to Rhode Island to play pickup basketball and talk football back then.

In 2011, O’Brien was working for the Patriots, and brought Chip in to explain his one-word play calls and no huddle offense to Belichick, who used it in the playoffs against the Tim Tebow-led Broncos. They both interviewed for the Eagles head coaching job when it came open in 2013. And so on.

You can tell how important Chip thinks Friday’s practice is because scheduled a light day Thursday to rest players up for it. Wednesday, he said:

“We have practice on Friday against Houston. So, after this one, tomorrow will be a little bit light and then we have a big one on Friday against Houston and that’ll be a big deal. And then find out exactly who’s healthy going out of that and after talking to our training staff, who’s going to be available to play on Sunday in that game.”

For the Niners, evaluating both wide receivers and cornerbacks has been difficult, because each position has major question marks. Have wide receiver problems been the result of great DB play, or have inferior pass catchers made the secondary look better than it is? Squaring off against Houston should help answer that question.