Many of the rules, formalized by Alexander Cartwright on September 23, 1845, were derived from earlier sources, but their compilation by the Knickerbocker Club provided a basis for modern adult organized baseball, in contrast to the various schoolyard ball games that had been around for generations previous.
As with James Naismith's famous original thirteen rules for the game of basketball, these early rules are notably spare, combining underlying "givens" as to the way the game is played with flexibility for circumstances. For example, the number of players on each team is not specified, nor is the number of innings, nor the distance between the pitcher and home plate.
Several of the rules are still around in some form today, while others are in direct contrast to current rules. A few of the more interesting examples are shown below. The list as presented, except for the commentary, is taken directly from the "Rules" as published in 1848:
- If a pace is taken to be 3 feet, that works out to 126 feet diagonally across the square that makes up the infield, or 89.1 feet between consecutive bases (the corners of the square). The rules currently specify the same method for marking off the bases, only at 127 feet 3-3/8 inches, which works out to 90 feet between bases.
8th. The game to consist of twenty-one counts, or aces; but at the conclusion an equal number of hands must be played.
- These original terms are recognizably card-playing jargon. The winner was the first team to score 21 "aces" (now called "runs", a cricket term), after an equal number of turns at bat or "hands". This rule, in combination with Rule 15, determined the length of the game in general. The game is now defined to be a certain number of "innings", another cricket term. In theory, a baseball game could be completed after just one inning, as long as one team scored the requisite 21 runs.
- The standard game length of nine innings was introduced in 1857. However, there are many circumstances in which baseball games, and variants such as softball, are shorter (or longer) than nine.
9th. The ball must be pitched, not thrown, for the bat.
- The ball had to be literally "pitched", like a horseshoe. Overhand pitching in baseball was not allowed until 1884.
- Note, however, that there was not yet a rule specifying precisely where the pitcher had to stand and deliver the ball.
10th. A ball knocked out of the field, or outside the range of the first and third base, is foul.
- A ball knocked between the baselines and beyond the field was not initially a home run but a foul, to be ignored (after finding the ball). This was largely a moot issue, as the early ball fields had very deep fences (if any) and an over-the-fence knock was an unlikely event.
- Foul balls were not initially "strikes". Some years later, when it became clear that a batter might hit foul balls endlessly in an effort to get a good pitch to hit, the pitcher was given a break by an 1858 rule that declared any foul ball to be a strike unless there were already two strikes on the batter. After the bunt came into existence as a strategy, it also became clear that a batter could literally bunt all day to try to get his pitch. To retain some balance, the rule was further amended, in 1894, to declare any foul bunt a strike.
- Note the colloquial term "knock", suggestive of the sound made when bat meets ball, and which is still used as a synonym; for example, a "base hit" is sometimes called a "base knock".
11th. Three balls being struck at and missed and the last one caught, is a hand-out; if not caught is considered fair, and the striker bound to run.
- "For it's one, two, three strikes, you're out!" is an ancient rule. The added detail, that a batter ("striker") can try to run to first on a missed third strike, also exists today, except that if there are fewer than two outs and first base is occupied, the batter is automatically out. This is to supersede the catcher dropping the ball on purpose to set up force plays - the same idea behind the Infield Fly Rule.
- Note the lack of reference to the strike zone or the concept of a "ball" or a "base on balls". Those adjustments developed over time to counter these strategies:
- Patient batters would refuse to swing at any pitch they did not like, and delay the game. The concept of the strike zone and the "called" strike was introduced in 1858.
- Over-cautious pitchers would throw the ball wide, and delay the game. The called "ball" (i.e. a pitch not a strike) was introduced in 1863, along with a limit on how many the pitcher could deliver, upon which the batter was automatically awarded first base. The number of balls constituting a "base on balls" was initially 3. It was tinkered with through the years (to as high as 9) until the count of 4 was settled upon in 1889.
- Foul balls were also not considered strikes initially, as discussed under rule 10.
12th. If a ball be struck, or tipped, and caught, either flying or on the first bound, it is a hand out.
- Catching a fair ball on the first bounce counted as an out until the 1865 season. Catching a foul bound for an out persisted until 1883. This was before gloves were used (or allowed), and obviously it was easier to catch that hard ball on the first bounce. This also provided the game with some balance, as the underlying assumption in Rule 8 is that many runs were likely to be scored. Also, the catcher played well back of the plate, for safety reasons, the various protective gear not having been developed yet.
13th. A player running the bases shall be out, if the ball is in the hands of an adversary on the base, or the runner is touched with it before he makes his base; it being understood, however, that in no instance is a ball to be thrown at him.
- The important part of the rule is not allowing a player to be put out by hitting him with the ball. This was sometimes called "soaking" the runner. One schoolyard version of the game, kickball, using a large inflated ball, still allows players to be put out by hitting them (below the head area) with this much-softer ball.
15th. Three hands out, all out.
- Three outs per half-inning, another ancient rule. Referring again to card-playing terminology, a "hand" is now called an "at-bat", or more generally, the progression of a specific batter and/or runner, at bat and/or around the bases.
- This is a fundamental difference from baseball's cousin, cricket, in which all of the batsmen take their respective turns at bat in a single innings.
20th. But one base allowed when a ball bounds out of the field when struck.
- "Ground rule single!" Outfields were assumed to be boundless, in general. The only "home run" was a literal dash around the bases, on a ball hit between outfielders.
- The Encyclopedia of Baseball, published by MacMillan, 1969, and subsequent editions
- Official Baseball Rules, various years
- The Dickson Baseball Dictionary, edited by Paul Dickson
- Baseball guides and annuals