The 1857 'Laws of Base Ball' was authored by former player Daniel 'Doc' Adams and established that bases would be 90 feet apart, games would have nine innings, and each side would have nine players.
Notes on verification
Multiple independent, high-quality sources (ESPN, MLB.com, Baseball Hall of Fame, SABR, Wikipedia) corroborate authorship by Doc Adams and the specific rules (90 feet, nine innings, nine players) with no contradicting evidence found. [cascade flags: wikipedia_dropped_other_sources_exist | tier=silver indep_score=0.787 clusters=4 claim_tier=notable]
Sources
- This Crackerjack Lineup of Baseball Memorabilia Drives ... (seed:culture_and_philosophy)
- https://baseballhall.org/discover-more/stories/pre-integration/adams-doc (corroboration)
- https://www.mlb.com/cut4/1857-laws-of-base-ball-sold-for-more-than-3-million-c174433764 (corroboration)
- https://www.espn.com/mlb/story/_/id/15155678/laws-base-ball-papers-sale-establish-new-father-modern-baseball (corroboration)
- https://robbreport.com/shelter/art-collectibles/baseballs-handwritten-rules-1857-may-knock-it-out-park-auction-231262/ (corroboration)