From BR Bullpen
MLB Rule 9.04:
- (a) The umpire in chief shall stand behind the catcher. ...
- (b) A field umpire may take any position on the playing field he thinks best suited to make impending decisions on the bases.
Beginning in 1952, four umpires are assigned to each major league game. The umpires work one series then move on to the next city, independent of any team.
During the regular season, umpires work home plate and the three bases. The second base umpire positions himself directly behind second base during play.
Umpires rotate one base for each game, i.e. the second base umpire today will work at first base tomorrow.
In amateur baseball the two umpire system is the most common, a home plate and a base umpire. The base umpire moves around according to the game situation.