RulesMLB Umpire Manual
First Basemen Playing in Foul Territory
Rule 5.02: Official Baseball Rule 5.02 provides that when the ball is put in play at the start of or during a game, all fielders other than the catcher shall be on fair territory. In particular, when holding a runner on first base, the first baseman shall position himself with both feet in fair territory….
2.01 Layout of the Field
The field shall be laid out according to the instructions below, supplemented by the diagrams in Appendices 1, 2, and 3. (1.04) The infield shall be a 90-foot square. The outfield shall be the area between two foul lines formed by extending two sides of the square, as in diagram in Appendix 1 (page 153)….