Luis Severino currently ranks second among all major-league pitchers in WAR. He’s third by park-adjusted ERA, he’s first by park-adjusted FIP, and he’s sixth by park-adjusted xFIP, and he’s done this while facing the seventh-toughest average opponent, among everyone with at least 50 innings. There was a time when a lot of the attention was on the development of Severino’s changeup. The changeup is there, and it’s not like Severino is afraid of it, but he’s blossomed into one of the best pitchers in either league largely on the back of his fastball and slider. Two pitches can be all you need, when said pitches are elite.

Just Tuesday, Severino blanked the Phillies over seven innings, whiffing nine without issuing a single walk. In all, 95% of his pitches were fastballs or sliders, and Severino’s 101st pitch clocked in at an even 100 miles per hour. It’s been said before that high velocity affords a pitcher a greater margin of error. That’s true, and part of the story behind Severino’s emergence simply comes down to how hard he can throw. Yet there’s more there, more that underscores how difficult Severino is for hitters. Let’s take a look at how his pitches play off of one another.

Here is that aforementioned pitch No. 101:

That’s how that at-bat opened. Here’s how that at-bat closed:

Not Severino’s first-ever three-pitch strikeout. After the game, Bill Evans heard from Andrew Knapp. I’ll excerpt the relevant quote:

Phillies catcher Andrew Knapp acknowledged it was difficult to pick up the slider coming out of Severino’s hand. The Phillies, who take pride in building up starters’ pitch counts, didn’t put Severino in any difficult situations until putting two runners on in the seventh. “It’s an electric fastball and you really have to honor that,” said Knapp, who was 1-for-3 with a double against Severino. “When he can throw that slider strike to ball, it’s tough to decipher which one is which. Kudos to him for going out and pounding the strike zone.”

There are some important nuggets in there. Knapp mentions the velocity, obviously. He also notes that Severino pounded the zone. This year, among all starting pitchers, Severino ranks third in fastball zone rate. Severino knows he has good heat and he uses it aggressively. He’s the rare flamethrower with better-than-average control. The other notable part of the quote is that Severino seemingly does a good job making his pitches look the same. Maybe that comes down to tunneling, or maybe that comes down to convincing arm action. But the whole point of pitching is to keep the hitter guessing, and Severino makes his fastball and slider tough to distinguish.

Now, Severino has had a fastball and slider from the start. He hasn’t always pitched this well with them. Consider this table of the best pitches thrown by any starters in 2018:

Based on raw run values, Severino has two pitches among the top ten. Based on run values per 200 innings, his slider is still there, in third place. Severino’s slider was very good last season — it had a run value of +17.9. He’s already topped that, in barely half the starts. We know enough to say that no pitch can be evaluated in isolation. A pitch is only as good as the other pitches a guy happens to throw. But Severino’s slider has continued to improve, benefiting both his best pitches. Here’s a look at how the slider has changed over time:

Spin rate isn’t a perfect measure, because there is such a thing as spin efficiency which raw spin rate doesn’t address, but what this makes clear is that Severino’s slider has evolved. He’s come to throw the pitch in a different way, to the point that, this season, his slider has more spin than almost anyone else’s. And that spin is translating into movement — Severino’s slider is almost like a curveball. Over time, he’s added both horizontal and vertical movement. And at last, this gets me where I wanted to go.

There’s one plot remaining. I looked at every starting pitcher this season throwing fastballs and sliders. On the y-axis in the plot below, you see average slider velocity. Easy. The x-axis is more complicated. For every pitcher, I found the average fastball movement, both horizontally and vertically. Then I calculated the total separation, in inches, between fastball movement and slider movement. You know what I mean when I talk about pitch separation, right? Severino is the point highlighted in yellow.

By slider velocity, Severino ranks in the 87th percentile. By fastball-slider separation, he ranks in the 92nd percentile. You see him there, kind of out by himself. It’s not just the velocity, and it’s not just the separation — it’s the combination of both of them. This, I think, is what helps to make Severino’s repertoire so lethal. Because he throws so very hard, hitters know they’ll have little time to react. But even despite that pitch speed, Severino can still make his fastball and slider go in very different directions. His average separation is just shy of 15 inches. For other pitchers who throw their sliders about as hard as Severino, their average separation is just about 10 inches. To put it another way, Severino throws his slider at 88.4mph. For other pitchers who get about as much separation as Severino, their average slider velocity is just about 82mph. Severino throws hard, but he still makes the ball move. That’s not very easy to do.

If a hitter just plain guesses wrong, he’s screwed regardless. That extra separation makes it harder to pull off a successful mid-swing adjustment. If you think fastball but get slider, or if you think slider but get fastball, you have to make a big change in very little time. It’s that much tougher to get the barrel to the right spot. When you’re talking about pitches and swings, a matter of inches can be an awfully big deal.

Luis Severino was already plenty good last season. I’m not sure he’s gotten that much better. In part, though, that just speaks to how well he already performed in 2017 over 31 starts. Over 17 more starts now in 2018, Severino has picked up where he left off, and then some. I don’t think there’s a starter with a better fastball-slider combination. Which is appropriate, because there are hardly any better starters, full stop.