How MLB Rules Changed From 1845 to Today

Baseball likes to sell itself as the sport that never changes. It is a good story and mostly false. The rules have been rewritten again and again since the 1840s, usually because one side of the game, pitching or hitting, got too far ahead of the other. The pattern is almost comically regular: scoring falls, crowds get bored, the rules committee moves something a few inches, and scoring comes back.

For bettors that pattern is more than trivia. Every major rule change reset the average number of runs in a game, which reset the totals books hang on the board. Below is the history by era, with the numbers where we could verify them from MLB’s own records, followed by what the changes mean for betting now.

The early game: 1845 to 1900

The Knickerbocker Base Ball Club of New York wrote down its rules on September 23, 1845, twenty of them in all. They are often called the Cartwright rules after club member Alexander Cartwright, although no documented evidence shows he wrote them. The important ideas are recognizable today: three outs to a side, foul territory, and a runner put out by a tag or a force instead of being hit with a thrown ball. An umpire was part of the setup from the start.

Most of the rest took decades. Early pitchers threw underhand from a box, batters could ask for a high or low pitch, and a walk took far more than four balls. The count kept being adjusted through the 1870s and 1880s until 1889, when four balls became a walk, and it has stayed there ever since.

The biggest single change came in 1893. Pitchers had been working from 55 feet 6 inches, and hitting was losing ground to the overhand fastball. The National League moved the pitcher back to a rubber slab 60 feet 6 inches from home plate. The league batting average jumped from .245 to .280 that season, and the distance has not moved since. It is odd to think that the most sacred measurement in the sport was a fix for low scoring.

The dead-ball era and the foul strike

Around the turn of the century the pendulum swung the other way. Until then a foul ball did not count as a strike, so a good hitter could foul off pitch after pitch until he got one he liked. The National League made fouls count as strikes (except with two strikes) in 1901, and the American League followed in 1903.

Strikeouts climbed and runs fell. Add a soft, dirty ball that stayed in play for most of a game, plus pitchers allowed to scuff, shine and spit on it, and you get the dead-ball era: bunts, steals, hit-and-runs and very few home runs.

1920: the spitball ban and the live ball

On February 9, 1920, baseball’s rules committee banned the spitball and other doctored pitches such as the emery ball and the shine ball. Seventeen pitchers who relied on the spitter were allowed to keep throwing it for the rest of their careers. The last of them, Burleigh Grimes, pitched until 1934.

At the same time, umpires started putting fresh, clean balls into play much more often, a change that got a push after Cleveland’s Ray Chapman was killed by a pitch in August 1920. A white ball is easier to see and a new one travels farther.

Babe Ruth is the best way to show what happened next. He hit 29 home runs in 1919, which was a record, then 54 in 1920 and 59 in 1921. The whole league moved with him. American League teams scored 4.09 runs per game in 1919 and 5.12 in 1921, and home runs across both leagues more than doubled over the same two seasons. That is the live-ball era, and it lasted, with ups and downs, for the rest of the century.

1968, the Year of the Pitcher

By the late 1960s pitchers had taken over again. Strike zones had been enlarged in 1963, mounds were high and inconsistent, and in 1968 teams averaged 3.42 runs per game. Bob Gibson finished the year with a 1.12 ERA.

The response for 1969 was blunt. The mound was lowered from 15 inches to 10, and the strike zone was shrunk to run from the armpits to the top of the knees. Four expansion teams arrived and each league split into two divisions, adding a new round of playoffs. Scoring jumped to 4.07 runs per team per game, an increase of roughly 19 percent in one season. It is the clearest natural experiment the sport has ever run on its own totals.

The designated hitter

In 1973 the American League let teams bat a designated hitter in place of the pitcher. Ron Blomberg of the Yankees became the first DH on April 6, 1973, and walked in his first plate appearance. The effect showed up in the scoreboard right away. American League teams scored 3.47 runs per game in 1972 and 4.27 in 1973, while the National League, still letting pitchers hit, went from 3.91 to 4.15.

For almost fifty years the two leagues played slightly different games. The National League finally used the DH in the shortened 2020 season, skipped it in 2021, and adopted it for good in 2022. If you ever wondered why older betting guides treated interleague games in National League parks as their own category, this is why.

Divisions, wild cards and a bigger postseason

The playoff format changed in steps. Divisions came in 1969. For 1994 each league moved to three divisions with a wild card for the best second-place team, but the players’ strike canceled that postseason, so the first wild cards played in 1995. A second wild card, with a one-game playoff, was added in 2012. Since 2022 the field has been 12 teams, six per league, with a best-of-three Wild Card Series.

The betting effect here sits in futures more than in single games. More playoff spots mean shorter odds on “to make the playoffs” and less value in a late-season push for a division that no longer decides whether a team plays in October. Teams that have clinched rest regulars in the final week, which is worth remembering before you bet a late-September game on the assumption that the starters will play nine innings.

The automatic runner

Extra innings changed in 2020. Every regular-season inning after the ninth now begins with a runner on second base, and the rule became permanent in 2023. It does not apply in the postseason.

For bettors the runner turns extras into short, high-scoring innings. A tie after nine is now far more likely to end in the tenth, and games that reach extras often push the total over. If you bet a full-game total, extras count, so the automatic runner quietly adds a little to the chance of the over in any game projected to be close. Our over/under guide explains how totals are graded when games go long or get cut short.

2023: the pitch clock package

The 2023 season brought the biggest set of changes in decades, all at once.

The pitch clock gave pitchers 15 seconds with the bases empty and 20 with runners on, and in 2024 the time with runners on was cut to 18 seconds. The average nine-inning game fell from 3 hours 3 minutes in 2022 to 2 hours 40 minutes in 2023, and to about 2:36 in 2024. There is more on that in our guide to how long a baseball game lasts.

The shift was restricted. Teams must keep two infielders on each side of second base, with both feet on the infield dirt when the pitch is thrown. Bases grew from 15 to 18 inches square, which cut 4.5 inches off the distance from first to second and from second to third. And pitchers were limited to two disengagements from the rubber, such as a pickoff throw or a step off, per plate appearance.

Put together, those rules brought back the stolen base. MLB’s own team totals show 2,486 steals at a 75.4 percent success rate in 2022, then 3,503 at 80.2 percent in 2023. The 2024 total was 3,617 at 79.0 percent. In 2025 it eased to 3,440 at 77.7 percent as catchers and pitchers adjusted. Scoring rose too, from 4.28 runs per team per game in 2022 to 4.62 in 2023, before settling at 4.39 in 2024 and 4.45 in 2025.

2026: the ABS challenge system

The newest change reaches the one thing nobody had touched: the umpire’s ball and strike call. In September 2025 MLB approved the automated ball-strike challenge system for 2026, covering spring training, the regular season and the postseason. Umpires still call every pitch. Each team starts the game with two challenges and keeps a challenge when it wins. Only the pitcher, catcher or batter may ask, immediately and without help from the dugout, and a team with none left gets one in each extra inning.

It is too early to say how much it will move scoring. What we can say is that the called strike on the edge of the zone, which catchers have spent years learning to steal, now has a limited appeal process.

What this means for betting

Rule changes move the average, and the average is where every total starts. When scoring jumped in 1969, 1973 and 2023, the first few weeks were the period when the market’s numbers were most likely to lag the real game, because the old data no longer described the new rules. That is the main lesson for the future: when a big rule arrives, early-season totals are built on history that just expired, and the books adjust as the data comes in.

Stolen base props are the clearest modern example. Before 2023 a steal prop on a fast player might sit around +250. After the bigger bases and pickoff limits, the same player’s chance of a steal rose, and books needed a few weeks to reprice. A $20 bet at +250 returns $70 including stake; at +180 it returns $56. Knowing that the league’s success rate went from 75.4 to 80.2 percent in one year tells you which way the price should have moved, and knowing it slipped back to 77.7 by 2025 tells you the easy edge is gone. Our odds calculators convert those prices into implied probabilities so you can compare them with a player’s real attempt rate. The prop bets guide covers how those markets are settled.

The pitch clock also changed how starters are used, which feeds straight into the first inning. Our NRFI rates track how often each team and starter keep the first inning scoreless under the current rules, and the probable pitchers board shows who is starting tonight. For how the lines themselves work, read our guide to baseball betting lines, and for the settlement rules that decide rainouts and suspended games, see the baseball betting rules page.

Bet an amount you are comfortable losing, whatever the rule book says this year. You must be 21 or older to bet in most US states, and 1-800-GAMBLER offers free, confidential help. Everything we track day to day starts on our home page.

Rule changes to watch next

The ABS challenge system is the one to follow through its first full season. Watch whether teams save challenges for late innings, whether catchers’ framing value drops, and whether walk rates move. Any of those would show up in totals before it shows up in the headlines.

After that, the list gets speculative. MLB tests ideas in the minor leagues first, and several of the 2023 rules spent years there before arriving. When a rule shows up in Triple-A for a full season, it is worth reading the league’s own reports on what it did to scoring, because that is the closest thing to a preview the market will get.