MLB

The league comprises 30 clubs, split evenly at 15 each between the American League and the National League, with the clubs based in the United States apart from one in Canada. Each league is divided into divisions, and the schedule is weighted so that clubs meet their own division most often.

One rule used to distinguish the two leagues: the American League permitted a designated hitter in place of the pitcher batting, while the National League did not. That distinction was removed when the designated hitter was adopted in both leagues, so a reader relying on older explanations will find them out of date. The two leagues now differ in branding and history rather than in that rule.

The regular season

Clubs play 162 regular-season games. Over a schedule that long, a slow start is recoverable and a hot month is not decisive on its own — which is what separates baseball's regular season from a shorter one in most other sports.

The postseason

The postseason involves 12 clubs, six from each league. That is an increase from the previous 10-club format, so an explanation written before the change will understate the field by two clubs.

Rounds progress from a wild-card series through divisional and league championship series, each played as a series rather than a single game, to the World Series between the two league champions. Because every round is a series, the postseason rewards pitching depth in a way a single-elimination bracket would not.

How the game is scored

• Innings. A game is divided into innings. In each, one team bats and the other fields; three outs end a team's turn.
• Runs. A runner who reaches home plate scores a run. The team with more runs after the final inning wins.
• Ties. A game level after the scheduled innings continues into extra innings until one team leads after a completed inning.
• Field dimensions. Unlike most sports, playing areas are not identical between stadiums. Outfield distances and wall heights vary, which is a legitimate factor in how a venue plays — and why some clubs are built around their home park.
• Pitch limits and rest. Pitching workloads are managed within a series and across a season, which is why starting pitchers do not appear every day.

Statistics worth understanding

Baseball is unusually statistics-dense, and a few measures explain most of what a casual reader needs.

• Batting average — hits divided by at-bats. Simple, but it ignores walks and the value of extra-base hits.
• Earned run average — earned runs allowed per nine innings pitched. A pitcher's measure, dependent on innings volume.
• On-base percentage — how often a batter reaches base by any means. Often a better guide to a hitter's contribution than average alone.
• Runs batted in — runs driven in by a plate appearance. Frequently cited, but strongly influenced by how often teammates are already on base.

These measures are stable definitions. Rule changes, by contrast, are not: pitch-timing rules, restrictions on defensive positioning and limits on pickoff attempts have all been introduced in recent years, so the playing regulations are worth checking rather than assumed.