Happy 10th birthday to the SABR/Baseball-Reference Encyclopedia. Thanks to all those who have contributed and welcome to any who wish to contribute.
From BR Bullpen
The Rules of baseball are the laws which govern how the game is played on the field. At almost all levels of play, from Little League to the World Series, the rules are based on the Official Rules of Major League Baseball.
The Official Rules are devised by the Official Playing Rules Committee of Major League Baseball, which is currently chaired by Sandy Alderson and includes current and former players, umpires and executives, plus an advisory member from amateur baseball, in recognition of the fact that the rules are used much beyond MLB or even the National Association of Professional Baseball Leagues. The committee makes yearly recommendations for changes or adjustments to the rules, announced at Baseball's Winter Meetings, although those are usually relatively minor, in contrast with other major sports. In fact, the last major change to the rules was probably the introduction of the designated hitter for the 1973 American League season.
The rules of baseball trace their history back to the New York rules established by Alexander Cartwright in 1846 for the first organized baseball game played on June 19 at the Elysian Fields in Hoboken, NJ between the Knickerbockers and the New York Nine. While many aspects of the game have changed since that historic day, the basic configuration of the rules remain, such as the nine players per team, the basic shape of the playing field and the layout of the bases, the alternance of innings after three outs, etc.
The development of the rules can be divided into four periods:
- The Early Days (1846-1870), when the game was largely played by amateurs, with no real central authority controling baseball's development. The rules tended to be fluid in those days.
- The Early Professional Rules (1871-1892). With the creation of the National Association of Professional Base Ball Players in 1871, there was a need for a first standardization of the rules, so that clubs from all over the country could play one another. However, pitching was still done underhand, first from a distance of 45 feet, then from 50 feet in 1881, while the number of balls required for a base on balls fell gradually from 9 to 4 in 1889.
- The Modern Pitching Era (1893-1919). With the pitcher's mound being moved back to its present distance of 60 feet 6 inches in 1893, the appearance of overhand pitching and the allowing of substitutions, the game started to take a shape very close to the modern one. There was also a rapid evolution in equipment, such as the introduction of fielding gloves and catchers' protective gear.
- The Live Ball Era (1920-present). The major change to the rules was the banning of the spitball and other unorthodox pitches as well as the modern definition of foul balls in 1920. The rules have been fairly stable since that time, with the major changes being the end of the bounce home run in 1931, the definition of the strike zone (which changed a number of times), and the introduction of the designated hitter in 1973. Other changes include the introduction of new statistics such as the sacrifice fly and the save.
 The Major League Rules
The Official Rules are divided into 10 chapters which cover the following matters:
- Chapter 1 - Objectives of the Game: the lay-out of the field, the ball, the bat, uniforms, gloves, batting helmets.
- Chapter 2 - Definition of Terms: definition of a number of terms used elsewhere in the rule book, in alphabetical order
- Chapter 3 - Game Preliminaries: Preparation of the field, game balls, substitutions, batting order, delays caused by weather
- Chapter 4 - Starting and Ending Games: Positioning of the players and coaches, ejections, how to score a run, length and score of a game, suspended games, doubleheaders, protesting games.
- Chapter 5 - Putting the Ball in Play: Live and dead balls
- Chapter 6 - The Batter: Batter's box, how a batter makes an out, batting out of turn, how a batter becomes a runner, designated hitter
- Chapter 7 - The Runner: Occupying a base, advancing from one base to the next, obstruction, how a runner is out, interference, runners out on appeal
- Chapter 8 - The Pitcher: Legal pitching delivery, forbidden actions by the pitcher (Doctoring the Baseball), warm-up pitches, balks, visits to the mound
- Chapter 9 - The Umpire: appointment of umpires, appeal of decisions, umpire-in-chief, report of violations to the League office, general instructions to umpires
- Chapter 10 - The Official Scorer: the official scorer, the official score report, runs batted in, base hits, stolen bases, sacrifices, putouts, assists, double plays and triple plays, errors, wild pitches and passed balls, bases on balls, strikeouts, earned runs, winning and losing pitcher, saves, statistics, guidelines for individual championships and cumulative performance records.
 Further Reading
- Paul Dickson: The Unwritten Rules of Baseball: The Etiquette, Conventional Wisdom, and Axiomatic Codes of Our National Pastime, HaperCollins Publishers, New York, NY, 2009. ISBN 978-0061561054
- Chris Landers: "10 bizarre baseball rules you won't believe actually existed", Cut4, mlb.com, May 22, 2015. [http://m.mlb.com/cutfour/2015/05/22/124363454
- Norman L. Macht: "Why Some Baseball Rules Need An Overhauling", Baseball Digest, December 1988, pp. 59-62. 
- Peter E. Meltzer: So You Think You Know Baseball ? A Fan's Guide to the Official Rules, W.W. Norton & Company, Inc., New York, NY, 2013. ISBN 978-0393344387
- David Nemec: The Official Rules of Baseball Illustrated, The Lyons Press, Guilford, CT, 2006.
- Wayne Stewart: You're the Umpire: 139 Scenarios to Test Your Baseball Knowledge, Skyhorse Publishing, New York, NY, 2010. ISBN 978-1602397934
- Glen Waggoner, Kathleen Moloney and Hugh Howard: Spitters, Bean-balls and the Incredible Shrinking Strike Zone: The Stories Behind the Rules of Baseball, Triumph Books, Chicago, IL, 2002.