Early evolution of the rules of baseball
From SABR Encyclopedia
 The Knickerbocker Rules of 1845
The key rules can be thought of as:
- The field is what we now call diamond-shaped, with 4 bases that is 30 "paces" apart
- A match is decided by which team has the higher score at the end of a full inning in which one team has reached 21 "counts, or aces"
- Three outs define a half-inning
- Fair hits are those within the 90-degree fair territory, with no base advancement by batters or runners on foul hits
- Runners cannot be put out by plugging them with thrown balls
- Three strikes retires a batter
- A ball caught on the fly or on the first bounce retires the batter -- the "bound rule"
- Deliveries to a batter are "pitches," not throws, implying underhand deliveries
Apparently, the custom of using teams of nine players in the early 1850s, but a formal rule was not adopted until 1858
Many common baseball terms, including "run" and "inning" are absent in these first rule, as are the size of a team, the pitching distance, and several running rules, including the idea of forceouts, tagging up, and stealing.
There is a reprint of the original set of Knickerbocker rules at 19cbaseball.com: Major Rule Revisions, 1845-1870
There is a more complete summary of rule changes from 1845 to 1883 at 19cbaseball.com.
 Later changes
- A game ends at nine innings; five innings define a legal game
- The pitching distance is put at 45 feet
- Runners cannot advance on caught flies
- A club should field nine players
- The umpire can call strikes when, after a warning, the batter declines to offer at good pitches
- The batter must hit from a specified zone
- Umpires can, after a warning, call balls; the batter is awarded first base, and other runners advance one base, after three called balls
- Fair balls caught on the first bounce are no longer considered outs