From the late 1940s through the early 1960s, Nino Escalera never went long between baseball games. The Santurce native played for minor league teams in seven states before making the majors with the Cincinnati Redlegs in 1954. He stuck with the Redlegs all season but batted .159, less than his listed weight. For the next eight years, he toiled in Triple-A; with the Havana Sugar Kings, Cincinnati’s Triple-A affiliate, he played outfield and first base in front of Fidel Castro. Each winter, he went home and started his second season, becoming a Puerto Rican winter league legend and eventually earning induction to the Puerto Rico Baseball Hall of Fame. After retiring, he scouted for the Mets and the Giants for almost three more decades. But for most who know his name, Escalera’s baseball life boils down to two plate appearances: one he made, and one he watched from the field.

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Escalera is primarily remembered for a first. When he made his major league debut, pinch hitting and singling for Cincinnati in the seventh inning on April 17, 1954, the then-24-year-old became the first black player in the franchise’s history. (The next batter, pinch hitter Chuck Harmon, became the franchise’s first African American player.) At 88, Escalera is the most senior of three surviving ex-players to have integrated an MLB club.

Escalera’s MLB career was limited to 73 games and 77 plate appearances for that 1954 team, getting most of his meager playing time as a pinch hitter, pinch runner, or defensive replacement. But before being demoted, he made more history, this time as the last to have done something. On May 22 in St. Louis, Escalera, a left-handed thrower, played shortstop, or so the box score said. No left-handed thrower has done so since.

With the Redlegs leading 4-2 with two out in the bottom of the eighth, the Cardinals’ Red Schoendienst singled off starter Art Fowler, bringing the tying run to the plate. The tying-run-representer was Stan Musial, whom Escalera describes by phone as “the greatest ballplayer in the league.” (Willie Mays might beg to differ.) Musial had singled in his previous two trips to the plate and had hit a pivotal grand slam in the game before. To prevent Musial from reaching scoring position by driving a ball into the gap, the Redlegs’ rookie manager did something drastic, and probably unprecedented.

“Birdie Tebbetts was the manager, and he made a shift on Stan Musial,” Escalera remembers. “The shortstop was [Roy] McMillan, and they took him out and they put me in to play right-center. And they left that left side open.”

Writing in the Sporting News on June 2, Tom Swope described “what probably was the first four-man outfield formation of its kind ever used in major league society.” The article included a diagram of the unorthodox defense, similar to a display Escalera says he has in his home to this day.

In becoming the last left-handed “shortstop,” Escalera had also become the first fourth man in an outfield (excluding baseball’s mid-19th-century formative years, when amateur proto-shortstops used to play shallow outfield by default).

“I had no idea it was coming until the game. And the game got so tight, you do it when the [situation] comes up. But it worked all right!” —Nino Escalera

Escalera, who says he liked playing for Tebbetts, describes him as “active” and “one of the intelligent baseball managers.” Perhaps echoing the ethos of a pre–Seitz decision era when the clubhouse balance of power was tilted more toward the manager than it is today—and when tactics were in some ways more malleable than they are today—he adds, “Whatever he said to me, I would do it.” In the weeks leading up to that game, Tebbetts, a longtime big-league catcher who’d exploit any edge, had schooled some of his players on the strategy, but Escalera wasn’t one of them. “I had no idea it was coming until the game,” Escalera says. “And the game got so tight, you do it when the [situation] comes up. But it worked all right!”

Musial was taken aback, too. In the book Birdie: Confessions of a Baseball Nomad, Tebbetts said, “Musial complained to the umpire, but there was nothing he could do about it,” and Swope’s account notes that first-base ump Dusty Boggess briefly halted the game until he was satisfied that Cincinnati didn’t have too many men on the field. Although there’s no way to know whether the extra outfielder affected Musial’s mind-set, Escalera says the alignment was “an advantage for us, in that situation.” And on that day, fortune favored the bold. Fowler, a knuckleballer, fanned Stan the Man on what Fowler told media was “the best fastball I’ve thrown in two years,” and the Redlegs went on to win.Tebbetts explained to Swope that he’d picked that day to deploy the shift because of Busch Stadium’s odd outfield dimensions. If Musial lined a ball off the park’s shallow right-center-field fence, one of the evenly spaced outfielders would have time to retrieve it and hold the lefty to a single. In a piece in the St. Louis Post-Dispatch about his “revolutionary brainchild,” Tebbetts said, “Since I couldn’t play a man on the roof, I did the next-best thing by adding a fourth outfielder to prevent the only other kind of hit that would have bothered me.” And he told the AP, “They don’t pay fellows like [Eddie] Mathews and Musial to hit singles in the clutch. I’m willing to let them bunt for a hit and give them first base. They can’t hurt you with a home run that way.”

As Escalera’s adventure reveals, a willingness to tinker with the standard spread of fielders in left, center, and right isn’t solely a 21st-century trait. Almost 64 years after Tebbetts hoisted his left arm to signal for a shortstop, though, the outfield is still baseball’s final defensive frontier. Four-man outfields remain a rarity, but modern managers are showing renewed interest in the tactic that Tebbetts pioneered. And as precise tracking technology enables increasingly accurate estimates of likely batted-ball locations, progressive players, coaches, and front-office analysts are continuing what Tebbetts began, redefining what outfields can look like in an experimental process that’s proceeding in earnest this spring.

Lately, infield shifting has lost a little of its revolutionary luster. After years of watching what once would have been singles swallowed by players arrayed to one side of the second-base bag, the standard overshift on pull-heavy hitters no longer qualifies as cutting edge, and watching teams toy with a slow-paced Albert Pujols is just sort of sad. Even the grounders that still sneak through the sides vacated by repositioned players aren’t eliciting the complaints from fielders and pitchers that seemed so routine a few years ago. After a continuous climb over several seasons, the use of infield shifts—which were roughly 11 times more common in 2016 than they had been five years before—actually declined slightly last year, according to Baseball Info Solutions. Although the shift remains roughly as effective as ever when batters hit the ball on the ground, hitters have grown more adept at elevating the ball when the shift is in effect, as FanGraphs’ Travis Sawchik has shown. Because line drives and fly balls tend to do more damage than grounders, more and more teams are turning their attention to optimizing their alignments against balls in the air.

Although the basic concept of shifting—stationing fielders in spots where they’re more likely to intercept balls—is the same everywhere on the field, outfield shifting is comparatively complicated. For one thing, fly balls tend to be more evenly distributed in direction than grounders, as revealed by the breakdown below of batted balls from last season.

2017 Batted-Ball Directions Batted-Ball Type Pull % Middle % Oppo % Batted-Ball Type Pull % Middle % Oppo % Ground ball 53.7 33.6 12.7 Fly ball 24 36.7 39.3

The less predictable the batted-ball tendencies, the fewer the obvious opportunities to take away hits. There are additional drawbacks unique to outfield shifts: For one, there’s more ground to cover in the outfield than the infield, and the potential cost (and embarrassment) of allowing a hit that otherwise would have been fielded is steeper; a gift from an infield shift usually results in a single, while a gift from an outfield shift might mean two to four bases. For another, the greater square footage and lack of landmarks in the outfield makes proper positioning more difficult on a fundamental level, requiring accoutrements like index cards, more memorization or, somewhat controversially, laser rangefinders combined with physical markings on the field. In the case of a four-man outfield, the defensive demands are greater, too; adding an outfielder without burning a bench spot (as Tebbetts did when he summoned Escalera) requires a rover with both infield and outfield experience.

It took plenty of time for infield shifts to catch on. In Birdie, Tebbetts credited the well-known overshift that Indians manager Lou Boudreau debuted against Ted Williams in 1946 for inspiring his own outfield alignment, but Boudreau may not have been the first to shift against Williams, and the concept dates back at least to the 1920s, if not to the 19th century. By the time Tebbetts began blazing trails in the outfield, Boudreau wasn’t the only infield shifter in a big-league dugout. The opposing manager in the series that featured the first four-man outfield, the Cardinals’ Eddie Stanky, was always in search of an edge, and habitually shifted against pull-oriented lefties like Mathews and righties like Ralph Kiner and Hank Sauer. In fact, the day after Escalera’s milestone moment, Stanky shifted against Cincinnati’s All-Star center fielder Gus Bell, Escalera’s right-hand man in the four-man outfield. Bell, a lefty, grounded a hit through the shift to left field, which kicked off a four-run flurry and forced Stanky to defend his decision. “As long as my figures show we’re better off, we’ll continue to shift,” he said to the Sporting News.

It took another half-century for infield shifting to spread, but once the post-Moneyball movement toward data-driven decision-making removed much of the front-office resistance, it quickly became ubiquitous. The four-man outfield, meanwhile, has disappeared and resurfaced every decade or two, often hailed as groundbreaking by observers who weren’t aware that it had happened before.

The ideal candidate for a four-man outfield is an extreme fly-ball hitter with pronounced pull tendencies on grounders but an all-fields fly-ball pattern, all of which is subject to change based on the ballpark, the pitcher, and even the count. Only so many combinations of matchups that fit the perfect profile, game states that call for a crackdown on extra-base hits, and managers daring enough to risk criticism in pursuit of a theoretical edge without a proven track record arise, especially given that field dimensions have grown more homogenous over time. It’s easy for skittish skippers to eschew extra outfielders by deferring to tradition. “I’m not that smart,” former manager Davey Johnson—himself a sabermetric innovator—said in 1999. “The way they invented the game, they had four infielders and three infielders.”

Yet following Musial’s strikeout, Tebbetts expressed his intention to keep using the strategy, and he did so sporadically against certain sluggers for the rest of his career. As late as June 1966, about two months before he managed his last game, Tebbetts deployed a four-man outfield against Senators slugger Frank Howard, explaining for the umpteenth time, “If I only overshift in the infield, he might get a double and be in scoring position. I put in the fourth outfielder, and I take away that double.”

In 1969, Reds manager Dave Bristol used a four-man outfield against the Giants’ Willie McCovey. So did Mets manager Gil Hodges, who also applied it to Dick Allen and Frank Robinson (who’d previously played outfield for Tebbetts). While with Texas, Whitey Herzog used a four-man outfield in the first and third innings against Harmon Killebrew in April 1973, and in May 1978, Herzog’s Royals used a four-man outfield four times against Jim Rice. And in 1999, multiple teams tried it against Mark McGwire, fresh off his 70-homer season.

Found this fantastic image in our archive, of the Reds shift against @SFGiants lefty Willie McCovey in 1969. Check where the SS is playing. pic.twitter.com/icIs69lDcV — Peter Hartlaub (@peterhartlaub) January 20, 2018

Since then, Joe Maddon has assumed the four-man mantel, putting the alignment into effect during his years with the Rays against lefty mashers like Travis Hafner, Jim Thome, and David Ortiz, and then brought it back last August against Joey Votto (who doubled), to much media fanfare. Although Maddon’s Cubs were the only team to try it in 2017, they weren’t the only one to consider it; buzz about the idea has been building, and at least two other teams seriously considered implementing it last season. (The midyear demotion of fly-ball extremist Ryan Schimpf removed the majors’ most vulnerable four-man-outfield candidate.)

The four-man outfield, then, is not close to new. Even the five-man infield in a non-walk-off situation, another reputedly boundary-breaking idea, has happened before; Dodgers manager Chuck Dressen tried it on April 20, 1951, against the Giants’ Artie Wilson, a speedster who supposedly never pulled the ball—and even Dressen borrowed the idea from an earlier minor league game. (Wilson’s major-league career, unsurprisingly, lasted 16 more games, in which he posted a .396 OPS.) A few years later, ex-Dodgers GM Branch Rickey introduced the idea to the Pirates. There’s been a lot of baseball.

What is different today is how teams are deciding to set up their defense. We know more today about the batted-ball tendencies of the 1954 Redlegs’ opponents than Tebbetts did at the time; Musial, it turns out, wasn’t an extreme fly-ball hitter in the mid-1950s, although Mays, another Tebbetts shift target, was. When Cubs manager Jim Riggleman rolled out a four-man outfield against McGwire in May ’99, he justified the decision by saying, “I keep charts, like everybody does, on where everybody hits the ball, and Mark, against us, has not hit ground balls to second. So we pulled a person out of that position.” If Riggleman was relying only on games that McGwire had played against his Cubs prior to the series, he was basing the move on a sample of only 49 at-bats. Today, teams have detailed data on the angle and hardness of hitters’ (and pitchers’) batted balls from the minors on up, with three years, minimum, of radar-derived major league measurements. No one need trust a small sample.

BIS doesn’t track outfield shifts, but that doesn’t mean they aren’t out there. Over the past couple of seasons, writers have reported Sasquatchesque sightings of more and more extreme three-man outfield alignments. In August 2016, the Astros almost abandoned right field against the Pirates’ Gregory Polanco, who hits his highest percentage of flies to left; last September, the Diamondbacks entirely abandoned left field against the Rockies DJ LeMahieu, a right-handed hitter who pulls very few flies. At least once, it worked.

According to Statcast data on outfielder starting position provided by Major League Baseball Advanced Media, the teams with the greatest combined variation in outfielder starting position last season, summing all three outfield positions, were—surprise!—the Rays and the Astros, both of whom were also among the top three in 2016. League-wide, there’s no evidence that the variation increased from 2016 to 2017 (2015 data isn’t available). But the top shifting teams may be getting more adventurous: The 2017 Rays, Astros, and White Sox all varied their outfield starting positions more than any team did in 2016.

Although teams will likely continue to push the outfield envelope along the same lines in 2018, even Astros-style shifts aren’t the newest new thing. The latest innovation isn’t moving outfielders one way or another, but asking them to trade places. “My guess is that within a couple of decades we’ll see corner outfielders routinely swap spots,” ESPN’s Sam Miller wrote after Maddon’s four-man-outfield defense against Votto. Make that more like seven months.

Over the past several years, proposing unorthodox defensive strategies has been a popular pastime on sabermetric sites. Many of the writers who’ve produced those articles have since gone on to get jobs with teams, working in capacities that may allow them to exert a tangible influence on their clubs’ in-game tactics. In 2012, Baseball Prospectus’s Max Marchi advocated bringing back the Tebbetts Shift; in 2014, the Indians hired him. In 2015, Chris Mosch chronicled incipient outfield shifting at BP; the following year, he went to work for the Angels. In 2016, FanGraphs’ August Fagerstrom broke down the best candidates for a five-man infield and four-man outfield; later that year, the Brewers hired him. And back in 2011, Beyond the Box Score’s Lewie Pollis made the case for what he called “adaptive defense”—alternating fielders between two positions based on the hitter’s handedness and batted-ball distribution, with the goal of assigning the superior fielder to the spot where the ball is more likely to be. Pollis is now a quantitative analyst in the Phillies’ R&D department—and not by coincidence, the Phillies are experimenting with mid-inning corner-outfielder swapping this spring.

“We now have plenty of evidence that shows why this could be a successful strategy, and we’re not afraid to show that evidence to our players.”—Sam Fuld, Phillies major league player information coordinator

Pollis, manager Gabe Kapler, and major league player information coordinator Sam Fuld—the latter two of whom are former MLB outfielders—have individually wondered about position-swapping for years, Fuld says via direct message. Now, the passage of time has brought them together on a team full of pliable players, in a period when players’ resistance to shifting has largely subsided. Fuld, a skilled defender who last played in 2015, says that 10 years ago, swapping corner outfielders would have seemed like a crazy idea. “Five years ago,” he continues, “it would have still been pretty nuts. Today it remains novel, but with the right personnel (younger, open-minded players) I think the swap can be executed without a lot of cynicism. Infield shifting has become so commonplace, so outfield maneuvering doesn’t seem as odd as it once did. Also, we now have plenty of evidence that shows why this could be a successful strategy, and we’re not afraid to show that evidence to our players.”

Fuld acknowledges that there’s an ego component to the outfield swap, especially because it isn’t established practice. Thus far, though, the Phillies haven’t encountered any resistance. “Our guys have been very receptive, so they haven’t even necessarily asked for proof that it may work,” Fuld says. “But we have it if they want to lay eyes on it at any point.”

Unlike the four-man outfield, which forces a team to risk ridicule for an expected benefit of fractions of a run, the value of season-long swapping could quickly add up—especially for the Phillies, who figure to have a big gap in defensive skill on days when the natural first baseman Rhys Hoskins and the speedy Aaron Altherr (who helpfully has experience in right and left, where fly balls break differently) are both manning corners. In 2013, BP’s Russell Carleton calculated that consistent corner-swapping could save a team close to half a win, and maybe more with the right personnel and analysis.

Although “nothing new under the sun” is the theme of allegedly unheard-of defense (at least since the ’50s), the “adaptive defense” idea might genuinely be without precedent if the Phillies pull off the plan with any regularity. The Baseball-Reference database contains only 15 games in which at least one outfielder switched corners more than two times, most or all of which don’t fit the Phillies plan’s profile. Only twice has the swap happened more than twice in one game, and both cases were desperate, injury-ridden, extra-inning games where a pitcher played the field. On May 14, 1988, Cardinals pitcher José DeLeón swapped corners with Tom Brunansky 11 times, starting in the 16th inning; DeLeón caught one fly in the 17th, but couldn’t track down the game-winning double two innings later.

“It took Cecil twice as long to get across the field as me. I was just nervous as a caged cat.”—Kelly Gruber, former Blue Jays second baseman

On occasion, we’ve seen similar extremes in the infield, but again only out of necessity. On consecutive days in 1959, the Indians’ Woodie Held and Granny Hamner swapped 16 and 17 times, respectively, at second and short when second baseman Billy Martin was hurt. And in the same month as the DeLeón game, Jimy Williams’s Blue Jays lost second baseman Manny Lee to a sore shoulder not long before first pitch. Working with what he had, Williams swapped starting second baseman Kelly Gruber and starting third baseman Cecil Fielder a record 18 times, based on the batter’s handedness. Gruber, the team’s regular third baseman, had some professional experience at second; Fielder, a first baseman/DH, had none. “It took Cecil twice as long to get across the field as me,” a laughing Gruber says by phone, adding, “I was just nervous as a caged cat … that wasn’t a position I was put in often.”

Last August, the Mets, who were without injured infielders José Reyes and Wilmer Flores, tied that 1988 record for single-game position changes by swapping starting second baseman Asdrúbal Cabrera and starting third baseman Travis d’Arnaud 18 times. Those outlier episodes, like Alfonso Soriano’s 10th-inning tour of the field in 2009, were last-ditch attempts to avoid disaster. The Phillies’ spring experiment is a carefully considered strategy to seek out success.

The irony of all of this optimization is that fielding opportunities are dwindling by the year. Thanks to rising strikeout and home-run rates, the average pitching staff last season allowed 405 fewer balls in play than the typical staff in 2003, a reduction of roughly 2.5 balls in play per game over 15 seasons. When it comes to designing the defense of the future, teams are fighting over smaller scraps, using similar stats. To outfield the field, they have to be fearless. “If I can figure out any move that I think will help my ball club win a game, I’ll use it,” Tebbetts said in 1955. “I don’t care whether it’s considered unorthodox. If it helps me, it’s all right.” In his day, that attitude may have made Tebbetts unusual. Today, it might make him mainstream.

Thanks to Martin Alonso of Baseball Prospectus, Daren Willman of MLBAM, and Hans Van Slooten of Baseball-Reference for research assistance.