Angels outfielder Mike Trout, Nationals outfielder Bryce Harper, Yankees outfielder Giancarlo Stanton and Tigers first baseman Miguel Cabrera are four of the most feared hitters in baseball. Between them, they have six MVP awards, 15 Silver Slugger awards, five league home run titles and 26 All-Star appearances.

This season, they have something else in common: Trout, Harper, Stanton and Cabrera are all wearing the C-Flap, that piece of protective plastic that attaches to a batting helmet and extends over the cheek and jaw.

Among these four power brokers, Stanton has been wearing the C-Flap the longest. After being hit in the face by a pitch that ended his 2014 season, he wore a football-style face mask in 2015 but switched to the C-Flap in 2016 and has continued to wear it since then, primarily against right-handed pitchers. (He goes flapless against southpaws.) Trout, Harper and Cabrera are more recent C-Flap converts, having begun wearing it just last month during spring training. While they're the highest-profile players to start wearing the flap, they're hardly the only ones. Players all over MLB have begun wearing the unassuming accessory, including other big names such as Boston's Hanley Ramirez and Houston's Carlos Correa.

As much as any player or team, the C-Flap appears poised for a breakout season in 2018. It's not just that more players are wearing it -- it's when and why they're wearing it. For decades, a player would wear a C-Flap only after he'd been hit in the face by a pitch, like Stanton, which is sort of like locking the barn door after the horse is already gone. Trout, Harper and Cabrera are part of a new wave of players who are wearing the C-Flap even though they haven't been beaned. They're wearing it to prevent an injury, not while rehabbing a past injury. One of those players -- Brewers outfielder Keon Broxton -- credited the C-Flap with saving his life after he took a pitch to the flap last season.

That shift in thinking is having a significant impact on the look of the game. As recently as two or three years ago, you could count the number of big leaguers who'd ever worn the C-Flap on two hands. Now barely a game goes by without at least one player wearing it. The net result is that MLB headgear, which has had essentially the same silhouette for decades, is undergoing a major visual shift. The C-Flap itself, after being on the market for more than 30 years as a rarely seen piece of specialized equipment, is suddenly becoming a standard component of everyday MLB gear.

But despite becoming more ubiquitous, the C-Flap remains curiously anonymous. Its inventor is virtually unknown even within baseball circles, and the flap is manufactured by a small sporting goods company that few people have heard of. No player has ever inked a deal to endorse the flap, which helps explain why most fans don't even know its name. (Be honest: Did you know it was called a C-Flap before you started reading this article?) In addition, due to an unfortunate confluence of circumstances, the C-Flap isn't approved for use in college baseball, high school baseball or most youth leagues, which has limited the product's growth potential even as it flourishes among the pros.

Most of this story has not been told before. With the C-Flap's use apparently reaching a tipping point, this is the perfect time to tell it.

Dr. Robert Crow is a retired plastic and reconstructive surgeon. Now 80 years old, he served as the Atlanta Braves' team physician from 1972 through 1999. "I was asked to put a lot of broken faces back together after they were hit with baseball," he said in a recent phone interview.

Crow had previously designed a few surgical tools, and in the mid-1970s, he began devising a face guard that would protect a batter's cheek and jaw. "I designed it in my kitchen, with Orthoplast -- the same material that's used for splints and casts," he said. "Then we proceeded to make molds and prototypes. I would take the prototypes with me down to spring training and ask some of the guys to try it out. They'd give me their feedback and suggestions, so we made modifications based on that."

By the early 1980s -- he's no longer sure of the exact date -- Crow had perfected his invention. The idea of a protective flap was not new (in fact, it long predated the idea of a batting helmet), but Crow's version of it -- a padded plastic flap that attached to the helmet via three screws -- was genuinely innovative. He called it a C-Flap, with the "C" standing partially for "Crow" and partially for "cheek." In 1987, he was granted a U.S. patent. By that time, he had set about the task of having the product manufactured and trying to get professional and youth players to wear it.

"I did not try to sell it to teams," Crow said. "But there's a group of major league physicians that meets every year. And at those meetings, I would tell my peers -- the doctors from all the other teams -- and let them know about the product."

Crow doesn't recall the first MLB player to wear the C-Flap, but the earliest adopter I'm aware of is A's catcher Terry Steinbach. After breaking an orbital bone due to a freak pregame accident in May 1988, Steinbach had facial surgery and returned to action about a month later wearing a C-Flap. His new headgear got a major showcase in the 1988 All-Star Game, when he hit a home run off Mets pitcher Dwight Gooden, and the flap got further exposure when the A's played in the 1988 World Series.

Steinbach, who's now retired and living in Minnesota, was unaware that he might have been a C-Flap pioneer. "For me, it was all about being able to get out there and play again," he said. "It was basically, if I wear it, I can play now. If I don't, I have to wait another three-and-a-half weeks. The bone was still healing [from the surgery], and the medical staff said they felt pretty confident that the flap would help protect it. It didn't feel weird or awkward at all, so I was fine with it."

Steve Vucinich, who was the A's equipment manager at the time (and still is today), had never worked with a C-Flap before. "It was not a common piece of equipment, and I remember we had to have a guy bolt it on there," he said. "Also, we couldn't get a flap that was green, so we had to go with black. Later on we painted it to match the rest of the helmet."

Terry Steinbach was the guy with the different helmet when he had his own moment in 1988. Focus on Sport/Getty Images

Steinbach continued going C-Flapped into the 1989 season, but that exposure wasn't enough to turn the flap into a mainstream MLB accessory. Over the next quarter-century or so, only a handful of players wore the flap. Those who did -- the list includes Braves outfielder David Justice, A's outfielder Terence Long, Indians infielder Kevin Seitzer, Cubs outfielder Marlon Byrd, Yankees infielders Charlie Hayes and Chase Headley and well-traveled outfielder Jason Heyward -- were coming back from facial injuries. None began wearing the flap simply for everyday protection. (The same is true of a separate group of players, some of whom played before the C-Flap was invented, who came back from facial injuries wearing a football face mask or something similar, including Pirates outfielder Dave Parker, Expos outfielder Ellis Valentine, Orioles outfielder Gary Roenicke, Twins outfielder Otis Nixon and Hayes when he was with the Rockies.)

Before the C Flap existed, players like Dave Parker had to basically raid the Steelers locker room for additional protection. George Gojkovich/Getty Images

The C-Flap probably would have gained more traction as a piece of everyday equipment if it had been widely adopted by colleges, high schools and youth leagues, but that has been problematic almost from the start. Most baseball governing bodies, such as the NCAA and the National Federation of State High School Associations, require that all equipment meet performance testing standards established by the National Operating Committee on Standards for Athletic Equipment (NOCSAE, pronounced "knock-see"). But NOCSAE has never established a standard for the C-Flap as a stand-alone item because, in NOCSAE's view, adding the flap to a helmet essentially creates a new helmet model, so the C-Flap would have to be performance-tested with every helmet on the market and with every new helmet model that came along in the future -- a prohibitively expensive proposition. Moreover, the various governing bodies, along with some helmet manufacturers, have taken the position that adding the C-Flap to a NOCSAE-certified helmet -- a process that usually entails drilling holes to hold the anchor screws -- constitutes a modification that voids the certification. The result is an odd catch-22 situation that might leave a reasonable observer wondering, "If it's good enough for MLB players, why isn't it good enough for kids?"

Crow, the Atlanta surgeon/inventor, eventually grew frustrated by his inability to penetrate the youth market and felt he had taken the C-Flap as far as he could take it. So in 2004, he sold the product to Markwort Sporting Goods, a small, family-owned St. Louis company, which has continued to manufacture and sell the C-Flap -- and has bumped up against the same catch-22 that Crow encountered.

"It's super-frustrating," said Herb Markwort, the company's CEO. "In fact, I just got an email today that says, 'I hope you can help me. At my son's high school baseball game yesterday, he was told by the umpire that he could not wear his NOCSAE-certified helmet because it had a C-Flap attached. The umpire said the C-Flap had to be NOCSAE-certified as well.' That's what we're up against. Fortunately, there are some common-sense coaches and parents out there who say the hell with the damn rules and just wear it anyway, even though it technically isn't legal."

If there's an upside to the NOCSAE situation, it's that the C-Flap -- whose original patent has expired -- still has the market largely to itself. "Nobody wants to invest in something that won't get NOCSAE approval, so we only have two very small competitors," Markwort said. "One is a little company called SST, but we don't see them as much of a threat. And there's another company that looks after the softball market, Boombah, but their product is just applicable to their own batting helmets."

But now, just as the C-Flap is catching on with MLB players, it might soon be facing its first real competition. MLB's helmet manufacturer, Rawlings, which currently buys C-Flaps from Markwort and supplies them to big league teams, is preparing to introduce a new helmet model this summer -- including the company's own protective face guard. The new product's name hasn't yet been finalized, but for now the working title is "R-Flap," for Rawlings.

In a potentially major development, Rawlings expects to get NOCSAE approval. "Our helmets will be certified," said Mike Thompson, Rawlings' executive vice president for marketing. "The holes will be pre-drilled. If the kid wants to just buy the flap, he can do that; if he wants it pre-assembled, he can do that. We've contemplated all those angles."

Markwort acknowledged that the Rawlings product would likely have a significant impact on the market. He also said that Easton, another helmet manufacturer, is working on a new helmet model with a protective flap. The flap wars might soon be upon us.

For now, though, the C-Flap is having its moment -- a moment that has been a long time coming. Aside from being offered in a wider range of colors, the product hasn't undergone any revisions, refinements or upgrades since Crow invented it more than three decades ago. Why is it suddenly so popular among big leaguers?

It's impossible to know for sure, but the breakthrough appears to have come in June 2016, when Cardinals catcher Yadier Molina began wearing the flap. According to a report toward the bottom of this page, Molina described his move as "a precaution" and said it wasn't prompted by any incident or injury. That was, to my knowledge, the first instance of an MLB player wearing the C-Flap proactively instead of reactively, and it seems to have gradually caught on from there.

Yadier Molina wasn't about to get hit in the face first and then decide he needed the C Flap. Jonathan Daniel/Getty Images

The C-Flap is also spreading throughout the minor leagues. In fact, the Brewers just purchased 500 C-Flaps and made them mandatory for their minor leaguers at the Single-A level and below. Players at the Double- and Triple-A levels are being strongly encouraged but not required to wear the flap, and a Brewers spokesman said "a large percentage" of those players are opting to wear it.

That raises an intriguing question: Could the day be coming when the C-Flap, or something like it, becomes mandatory at the big league level? After all, batting helmets themselves were once optional, and then they became mandatory. Then came the earflap, which was optional until it too was made mandatory (and then double-earflaps became mandatory in the minors). When viewed in that context, a face guard flap could be the latest step in the evolution of standardized MLB headgear.

"I can definitely see it becoming mandatory," said Thompson, the Rawlings executive. "You want to protect the assets, right? There's really no downside to it, and now you have the best guys using it." Most other people interviewed for this article agreed that some sort of protective flap could eventually be required, though some of them cautioned that these types of rule changes are complicated by the collective bargaining process between MLB and the players' union.

In the end, it might not matter. If the current trend continues, flapped players -- whether they're wearing the C-Flap or a competing product -- might soon outnumber non-flappers. If other teams follow the Brewers' lead and make the flap mandatory in their minor league systems, an entire generation of players will come of age wearing the flap, so there's a good chance they'll continue wearing it in the majors.

In other words, what we're witnessing right now is likely a key moment of transition in the look of the sport. That isn't something that comes along very often. Enjoy watching it as it unfolds.

Paul Lukas, a lifelong Mets fan, still remembers that Terry Steinbach home run off of Dwight Gooden like it was yesterday. If you like this column, you'll probably like his Uni Watch Blog, plus you can follow him on Twitter and Facebook and sign up for his mailing list so you'll always know when a new column has been posted. Want to learn about his Uni Watch Membership Program, check out his Uni Watch merchandise, or just ask him a question? Contact him here.