The manager of the home team shall present to the umpire-in-chief and the opposing manager any ground rules he thinks necessary covering the overflow of spectators upon the playing field, batted or thrown balls into such overflow, or any other contingencies. If these rules are acceptable to the opposing manager they shall be legal. If these rules are unacceptable to the opposing manager, the umpire-in-chief shall make and enforce any special ground rules he thinks are made necessary by ground conditions, which shall not conflict with the official playing rules.
Double off catwalk, fair and playable under ground rules
Universal Ground Rules and Ground Rules specific to individual parks
Umpires misunderstand ground rules and award a home run