Rule 4.05

Rule 4.05
(a) The team at bat shall station two base coaches on the field during its time at bat, one near first base and one near third base.
 
(b) Base coaches shall be limited to two in number and shall (1) be in team uniform, and (2) remain within the coach’s box at all times.

PENALTY: The offending base coach shall be removed from the game, and shall leave the playing field.

Rule 4.05 Comment: It has been common practice for many years for some coaches to put one foot outside the coach’s box or stand astride or otherwise be slightly outside the coaching box lines. Until a batted ball passes a coach, a coach is not permitted to position himself closer to home plate than the coach’s box nor closer to fair territory than the coach’s box. Otherwise, a coach shall not be considered out of the box unless the opposing manager complains, in which case the umpire shall strictly enforce the rule and require all coaches (on both teams) to remain in the coach’s box at all times.

It is also common practice for a coach who has a play at his base to leave the coach’s box to signal the player to slide, advance or return to a base. This may be allowed if the coach does not interfere with the play in any manner.


DISCUSSION
The teams must have two base coaches occupying the coaching boxes. The umpire may not begin a half inning until both coaches have reached their boxes. [PBUC]

The base coaches must be in uniform and may not wear jackets over the uniform. [MLBUM]


HISTORY

In 2008, MLB initiated the new rule requiring base coaches to stay behind the front line of the coaching box until a batted ball had passed. Los Angeles Dodgers' third base coach Larry Bowa refused repeated warnings to stay behind the line and was ejected.
http://mlb.mlb.com/news/article.jsp?ymd=20080402&content_id=2483998&vkey=news_la&fext=.jsp&c_id=la