Let’s start with the obvious: Defense hasn’t been the big surprise of Marcus Semien’s 2019. That story was written a few years ago. The big surprise of Semien’s 2019 has been that in consequence of a .373 wOBA and 7.5 WAR, he has precisely doubled the value he produced last year in the second-best season of his seven-year career. I wrote about Semien’s offensive breakout in May, noting his much-improved plate discipline, and suggesting that its probable cause — a change in approach at the top of the strike zone — augured continued success. Dan Szymborski picked up the thread in July, finding much the same, and also discussing Semien’s defense at some length. Dan concluded:

[D]efensive numbers are volatile, so having a second year of improved defensive numbers significantly betters the chances that Semien’s reinvention with the glove is for real. That the improvement is largely driven by error rate is an even more promising development because though errors themselves aren’t a great measure of defense, error runs tend to be more predictive on a year-to-year basis than range runs. This isn’t surprising given that range numbers necessarily have to jump into evaluating theoretical plays that never happened. In error runs, Semien’s +4.6 ties with Paul DeJong for the best in baseball at any position in 2019. If he continues on this pace, he will have added roughly 20 runs compared to the 2015 season, simply from avoiding errors.

Going into the final day of the season, Semien was +6.9 ErrR, which put him 19.5 runs ahead of 2015’s abysmal mark, and contributed no small amount to his overall WAR figure (which, incidentally, Dan optimistically projected for six runs in July, and which Semien blew past in August). In this piece, I want to pick up on something a commenter on Dan’s piece pointed out: that the major improvement in Semien’s defensive numbers came not right after his much-ballyhooed 2015 heart-to-heart sessions with Ron Washington, but over the course of 2017 — right when Matt Chapman became the A’s everyday third baseman.

Marcus in the Field Year ErrR DRS UZR/150 2015 -12.6 5 -12.2 2016 -3.5 -6 -5.5 2017 0.5 -9 -6.2 2018 0.0 9 6.4 2019 6.9 3 5.0

Chapman took over for good halfway through the 2017 season, and the numbers diverge some overall, so I hesitate to put too much heft behind Chapman’s position at the hot corner as the sole explanation for Semien’s improvements. Semien himself clearly valued Washington’s instruction, and it’s certainly possible that it took a solid year or so to put the lessons fully into practice. And whatever benefit playing next to Chapman might have given Semien (we’ll get into that shortly), it was clearly easier to take advantage of because Semien was a better defensive player in 2017 than in 2015. This is a “both and” situation. Still, the dynamic interested me enough that I asked Semien and Matt Williams, the A’s third base coach, about it last week. They both saw something there.

“I think the deeper he [Chapman] can play, the closer to straight up I can play,” said Semien. “And that really helps me going right to left. I think he has the ability to play so deep because he reads guys who are bunting and their mannerisms before they do it. He has a good sense of that. He’s really good at reading hitters and reading what’s different about this, so even when playing way back he’s already on the bunt. It’s been a good combination these last two years.”

Semien told me what should be obvious to anyone who’s watched him play defense: That he’s more comfortable going to his backhand (towards third base) than his forehand (towards second). Chapman’s range and overall skill at the position to his right — his 8.3 ErrR this year is first to Semien’s third — mean Semien has been able to play far closer to the second base bag than he had earlier in his career. That makes plays to his left — up the middle — much more within range, and allows him to stretch into the no-man’s land between him and Chapman on his strong hand.

“Chappy helps him because he covers great ground,” said Williams, who joined the A’s at the beginning of the 2018 season. “At the same time, Marcus has become an elite shortstop because he’s paid attention to the details. Not just catching the groundballs. He’s past that. He’s past having any confidence issues with regard to throwing or catching it or anything like that. Now he can just simply play, and he’s taken it to the next level where he’s actually anticipating. Over the last couple years, he’s really taken to that. Understanding hitters, understanding our pitching staff, anticipating a pitch that’s thrown, and being in the right spot.”

Inside Edge suggests that in 2019, Semien has achieved career-high success rates on Even (40-60%), Likely (60-90%), and Routine (90%+) plays. Meanwhile, at least in part due to his positioning closer to the middle of the field, more of the groundballs he’s faced have fallen into one of those three categories than ever before: 92.2%. Some of that is just selection — balls in the hole that previously might have fallen into the Impossible or Remote categories aren’t even within the realm (or reach) of possibility any more, and fall within Chapman’s remit. It doesn’t seem to have hurt the A’s team defense overall; they’re still, by any reasonable metric, among the best defensive units in the game.

“I think that since Chappy has been here, both of their confidence levels have risen,” said Williams. “That means that they’re certainly comfortable playing next to each other, they communicate well. They pick each other up with regard to a grounder that is hit to that side. They’re both very studious in the scouting aspect of it. They understand the hitter, the pitcher, and what they need to do defensively. But most of all, they take great pride in it and understand that the reason that we are where we’re at is, granted they’ve both got 30+ homers, is because they’ve played great defense. The little things, they’re very attentive to. It comes from a lot of work. Lots of work. And understanding what he can accomplish. There’s a reason he’s turned himself into an elite shortstop and it is that he doesn’t have to worry about the minutiae of that.”

Semien, for his part, is pleased but not satisfied with his success in the field and at the plate. “I’d like to keep working on throwing, finishing my throws, no matter what grip I end up in my hand, knowing that you need to finish your throws so that the ball stays through. Catching the ball I’ve felt pretty good, but throwing is what I’ve been up to this year.” Anyone who’s watched Matt Olson stretch to rescue a Semien throw will know that. But then, that’s the same story again: These A’s can catch it and they can throw it, and they do it as a team. They’ve won 97 games two straight years, and later this week they have a chance to finally break through the one-game playoff and play their first full postseason series since 2013. Team defense will be a big part of the reason why.