The backstory: Over the course of the PITCHf/x era, the major-league strike zone kept getting bigger and bigger. Last year, for the first time, the zone basically stabilized. The commissioner floated the idea of raising the zone’s lower boundary. The union disapproved. In theory, this year’s zone should look familiar. It should look like last year’s zone.

And, you know, it does look like last year’s zone, in that the strike zone never changes that much. Even the dramatic changes are actually subtle changes. But here are two league heat maps, showing called-strike probabilities. One of these reflects the 2016 season, and the other reflects 2017 so far.

Changes! Somewhat substantial changes. Some of them actually a little too substantial. One is reminded that the tracking systems have changed this year, and that affects more than just velocity readings. All the data from before 2017 came from PITCHf/x. Now we’ve got Trackman data instead. They might report slightly different pitch locations, which could explain the heat maps above. So as a proxy, how about something else? How about something as simple as…strike rate?

Two years ago, 64% of all pitches were strikes. So far this year, that’s down to 63%, and while a change of one percentage point isn’t going to render the game unrecognizable, you can see the clear trend leading up into 2015. Strikes were on the rise. Now they’ve gone in the other direction, and this year’s early (EARLY!) rate is right in between where the league was in 2010 and 2011. It doesn’t necessarily directly reflect a change to the strike zone, but it’s suggestive. Walks happen to be up, and Dave wrote about that earlier Monday.

For reasons unbeknownst to me, Baseball-Reference reports numbers that are even more striking. According to them, over the past three years, strike rate has slipped from 64.3% to 63.8% to 62.7%. Swing rate is also a little down. The zone could be smaller. The pitchers could just be throwing more pitches out of the zone. Could easily be both. You can’t expect any actual exhaustive research a week into the regular season.

Here’s where we are: Walks are up, and unsurprisingly, related to that, strikes are down. We’re not used to seeing strike rate going down! This is absolutely something to watch, because even as early as it is, there have still been plenty of games. Something could be happening. Recent trends could be turning around.