2017 was the year of the immaculate inning in MLB. This is why.

I did not know what an immaculate inning was until very recently. One possible reason for that: They almost never happened until very recently! An immaculate inning is when a baseball pitcher completes a half-inning in exactly nine pitches, all of them strikes. Three guys up, three guys down by strikeout. It’s very satisfying. It’s also very rare, but ...

Immaculate innings are on the rise!

There’s a 25-year stretch between 1928 and 1953 in which no pitcher threw three straight three-pitch strikeouts in an inning. It happened three times in an eight-day stretch of August 2017. So yeah, immaculate innings are up a whole lot. But why?

Strikeouts are on the rise!

Hitters strike out more now than ever. MLB is on a streak of setting a new all-time record for averages strikeouts per nine innings each season. It makes sense that, if strikeouts as a whole are way up, immaculate innings would be up, too.

But why are strikeouts up?

Strikes are on the rise!

I imagine there are several reasons, but I like this Ringer piece on some recent theory under the name Three True Outcomes (TTO). The thinking goes: With some exception, a pitcher can only affect three outcomes of an at-bat: walk, strikeout, and home run. The fate of any ball batted into the field is up to defense and base-running.

Pitchers throw harder these days, but TTO for our purposes is particularly important as it relates to hitters. More and more MLB players step up to the plate thinking that a strikeout — while humiliating — is hardly worse than any other kind of out. A home run, though, is far preferable to any other kind of batted ball, on account of the thing where they give you a whole run for it. Therefore, why not swing, and swing for the fences? Worst case is you whiff thrice, but worst case is always some version of an out. Thus, strikeouts are up. Homers are up, too (though many would argue there are more reasons for that.)

That’s the vibe of this era in MLB history, and it’s perfectly encapsulated by the rise of an exciting statistical oddity nearly unheard of before: the immaculate inning. Look out for ’em!