Though governed by men whose emotional stability historically has been questioned every 10 minutes, baseball's rules structure has remained remarkably steady for more than 100 years. While basketball fiddles with 3-point lines and football puts its pass-interference, overtime and ref-upstairs rules in a Cuisinart each offseason, baseball rules remain as suggestible as a glacier.

Nonetheless, the game was not always this way. Beginning with Alexander Cartwright's Knickerbocker rules in 1845 that set the bases 90 feet apart with three outs per side per inning, baseball spent its first 50 years lurching and grasping for the balance that has kept so well ever since. Even after finding it, plenty of other alterations have shaped the sport we know today.

Herewith are the top dozen rules changes (both on-field and procedural) in baseball history. We won't go back so far as the barbarian days when "soaking" -- i.e., pegging him with the ball -- put the runner out, or recount every substantial change before 1900, leaving the 20th century with a fighting chance:

ALAN SCHWARZ'S TOP DOZEN

1 The mound moves back (1893) Baseball spent most of 1850-90 changing the pitcher's role from simple deliverer of offerings for the hitter to hit to a legitimate weapon of the defense. Restrictions were gradually lifted on curving pitches (1872), overhand deliveries (1883-84) and running starts (1887). The last major change took place in 1893, when instead of delivering the ball from a 50-foot line the pitcher had to keep his back foot on a rubber slab 60 feet, 6 inches from home plate. The resulting five- or six-foot lengthening of pitching distance sent offense through the roof for a few years but soon thereafter afforded the equilibrium we know today.

2 Abolition of spitball (1920-21) Until this time, baseballs were allowed to be slathered with spit and discolored with everything from tobacco to licorice juice, even by infielders, all to keep the batter from getting a good look at the ball. Teams in 1920 were allowed to designate only two spitball pitchers apiece -- imagine Gene Orza's grandfather's reaction to that one -- but the long-running debate continued before the seminal moment arrived: a pitch from Yankees submariner Carl Mays, not believed to be a spitball but still a dreary, brown ball that was hard for the batter to pick up, crashed into the skull of popular Indians shortstop Ray Chapman and killed him. Rules were changed the next offseason to allow only 17 pitchers (whose livelihoods were in the balance) to continue throwing spitballs, and umpires were instructed to introduce new white balls far more often. Several factors helped usher in hitter's offensive boom of the 1920s; being able to actually see the ball was one of the biggest.

3 Free agency (1975) Perhaps you've heard of it. Marvin Miller triumphed over the unconscionable reserve clause, and we've barely heard from the union since.

4 Modern substitution allowed (1891) As strict as baseball is regarding substitute players -- imagine the insipid specialization that would follow if it followed football and basketball and allowed subs willy-nilly -- it once was even more stringent. As late as 1889, only one replacement was allowed per game. It went up to two for one season before being eased into its modern form, allowing free substitution but with no re-entry into the game. The world was safe for pinch-hitters, relief specialists (who weren't already in the game at another position) and Charles Gipson.

5 The foul strike rule (1901 and 1903) Foul balls didn't count as strikes until 1901 in the National League and 1903 in the American League. (Ever wonder how Nap Lajoie hit .426 for the 1901 Philadelphia Athletics? There's your answer.) After this rule's adoption, offense plummeted, strikeouts zoomed and a pitching-dominated decade was born.

6 The designated hitter (1973) First discussed in the 19th century and then more seriously by National League president John Heydler, the AL originally excised pitchers' hitting only on a three-year experimental basis, making it permanent in December 1975. The AL in '72 was lagging behind the NL in runs per game by a whopping 6.9 to 7.8, but immediately vaulted to 8.6 and hasn't looked back despite purists' calls for the contrary. Several forests have lost their lives to this ensuing and incessant debate.

7 Game-winning hits (1920) What fun was it before this rule, when a hitter, down 4-3 in the bottom of the ninth with a man on third, blasted a ball 430 feet over the fence and was credited with ... a single? (Sound weird? Fair balls before 1931 that bounced over or through a fence went down as homers. Figure that out.) Babe Ruth lost one homer to this rule, as did Frank "Hey, I Really Did Hit a Home Run" Baker.

8 The strike zone (various years) That pesky little rectangle has been adjusted half a dozen times since umpires began calling strikes in the early 1860s, including 1950 and 1963. But the most significant change to modern fans came in 1969, when, after a soporific period of 2-1 scores and .238 batting averages, the zone was shrunk from top-of-shoulder, bottom-of-knee to armpit, top-of-knee. (The mound was also lowered from 15 to 10 inches, though pitchers and batters I've spoken to from that era claim that change's effects have been blown out of proportion.) Following a generation-long corrosion of the rectangle, with different umpires defining it however they saw fit, Sandy Alderson's current attention to the strike zone is one of the best things to happen to the game in a decade.

9 Expansion (1961-62) Following the threat of a new Continental League and the breaking of tradition through club moves to Milwaukee, Baltimore, Los Angeles and San Francisco, baseball slapped itself awake by expanding its little cartel from 16 teams to 20. Season adds eight games. Roger Maris' head loses eight thousand hairs.

10 Four balls, three strikes (1889) Umpires began calling balls and strikes in 1863. By 1874 a walk was nine balls, but it dwindled down to four by 1889. One year, 1887, saw batters granted four strikes. Imagine how long A's and Yankees games would have taken under these old rules.

11 Hoodlumism go home (1901) Not sure where to start this, so I put it at 1901, when Ban Johnson's more reputable American League began to compete with the rough-and-tumble National, which had spent the previous decade making the South Bronx look downright effete. Sorry, but I don't long for the old days when John McGraw would grab opponents' belts while the one umpire tracked a fly ball and Mike Kelly would throw his catcher's mask in the way of runners trying to score. "Hoodlumism and dirty playing ... were worse than ever," historian Harold Seymour wrote of the 1890s. "There was scarcely an issue of The Sporting News that did not tell of kicking and wrangling with umpires, fights among players, indecent language, and incidents of rowdyism in general."

12 Ballpark dimensions (1959) After a century of weird parks that made the game too dependent on playing conditions -- Polo Grounds fences were 250-280 feet down the lines, while Braves Field's left-field wall stood 403 feet from the plate, to name a few -- parks after 1959 had to have minimum distances of 325 to the foul poles and 400 to center field. Unfortunately, far too much uniformity followed, with parks such as Three Rivers Stadium and Veterans Stadium that brought the ambiance of a hospital ward, but the modern park revolution let the game have some fun again.

Alan Schwarz is the Senior Writer of Baseball America magazine and a regular contributor to ESPN.com.