Minor League Baseball finally returns to ballparks across the country, with Tuesday marking the first games since 2019 after all of last season was canceled because of the COVID-19 pandemic.

Opening Day will feature 55 contests across four levels, though not every team that took the field two years ago will be returning to the diamond in 2021.

Major League Baseball streamlined the minor leagues, contracting 40 teams in the process. All 30 MLB teams will now have four levels of affiliates: low-A, high-A, Double-A and Triple-A -- plus a rookie level team housed at each spring training complex.

The post-pandemic restart will also look quite different on the field as baseball experiments with different rule changes at every level, as well as in the independent Atlantic League, as part of an ultimate goal of finding ways to improve the sport. Strikeouts are up, contact is down, games are longer than ever and MLB is aiming to do something about it.

Former Chicago Cubs and Boston Red Sox front-office exec Theo Epstein has been charged with overseeing the experiments for MLB. He will be analyzing the data and watching games closely.

"There's a whole team at MLB that is thinking through how to collect the right kind of data, how to analyze the data and dissect it in ways that will allow us to understand the impacts of the rule changes," Epstein said recently in a phone interview. "We also want to understand how they interrelate to one another and make sure we're avoiding unintended consequences."

So with that in mind, and with some help from Epstein, let's examine the major rule changes you'll be seeing as you attend and watch Minor League Baseball in 2021.

Triple-A: Larger bases

Bases will increase from 15 square inches to 18 square inches. That also means the distance between the bases will be shortened slightly. It's a small change but the league thinks it could have an impact in various ways: reduced injuries, more baserunners on bunts and soft contact due to a shorter distance to the bag and even more players attempting to stretch singles into doubles. And of course more stolen base attempts. In other words, just more action.

"Just putting the ball in play will make you more likely to reach the base," Epstein said. "Infields would have to play in. More balls would get through. The value of a single rises dramatically. The trip around the bases is easier. Now there'll be a huge premium on just getting on base and athletes who can run.

"You're not necessarily accomplishing all that by extending the base three inches but you are around the margins. You are nudging the game in a better direction."

Double-A: Regulating the shift

To start the season, infielders will be required to have their feet on the dirt, though they can stand anywhere on the infield.

In the second half of the Double-A season, the rules will require two infielders to be positioned on either side of second base as well as have their cleats on the dirt as the pitch is thrown.

Baseball is still figuring this one out, hence breaking up the experiment to the first and second halves of the season.

"There are some issues to going to two on each side," Epstein explained. "There's enforcement and player safety. You might have fielders moving with the pitch or the swing. Umpires would have to be focused on the action. A little more work to do."

Low-A Southeast: Automated balls and strikes (ABS)

Robot umpires will be experimented with for the first time in affiliated baseball. Previously, the Atlantic League and the Arizona Fall League used electronic signaling for balls and strikes, but now select low-A games will have the home plate umpire wearing an earpiece connected to TrackMan radar systems installed in the park. The software will say ball or strike to the umpire who announce it to the players and crowd.

Epstein stressed electronic calling of balls and strikes isn't just about umpires getting the call right.

"You're seeing the ABS being used in the low minors this year because with that comes the potential to change the strike zone to one that is optimal for contact," Epstein said. "Different strike zones lead to different styles of play."

Atlantic League: Moving pitching rubber back a foot

This change, debuting in the second half of the league's season, could be the panacea baseball is looking for without changing the aesthetics of the game. Will anyone really notice the rubber is 61 feet, six inches from home plate instead of 60 feet, six inches? The last time baseball moved the rubber back -- granted it was five feet and in 1893 -- strikeouts declined and batting averages went up 35 points.

"The extra foot gives the hitter an extra 1/100th of a second of reaction time, which is the equivalent of a mile-and-a half of velocity," Epstein said. "The presumption is that it will allow hitters to make more contact against premium velocity. That's the theory."

More new rules

• At high-A, pitchers must disengage the rubber prior to throwing to any base or else a balk will be called. With this change, Left-handers will no longer be able to step towards first base with their foot on the rubber. The goal is an increase in stolen base attempts.

• At all low-A levels, pitchers will be limited to a total of two "step offs" or "pickoffs" per plate appearance while there is at least one runner on base. A pitcher may attempt a third step off or pickoff in the same plate appearance, however, if the runner safely returns to the occupied base, the result is a balk.

• At Low-A West, a 15-second pitch clock will be implemented with on-field timers expanded to one in the outfield and two behind home plate, between the dugouts.

• The Atlantic League will experiment with the "double-hook" designated hitter rule. Once a team's starting pitcher is removed from the game, so is their designated hitter. The goal is to incentivize starters to go along as well as create late game strategy.

Fan polls, hours of deliberations and analyzing data have landed baseball at this moment in time: experimenting with a game that's well into its second century of existence. Change is inevitable. But of what kind and how much? The 2021 minor league season will help determine the future of the sport.

"We have to do it in a way that isn't too far removed from the essence of baseball," Epstein said. "No one is looking to reinvent the wheel here. This is the greatest game in the world and we want to reserve the essence of baseball. A lot of this is restoring the game to the way it's historically been played."