There are 76 active major league umpires as of 2021.
Those arbiters are divided into teams of four umps each, with each grouping designated with a single letter of the alphabet, ranging from A through S — 19 teams in all.
For any given game, the umpires man home plate (who calls balls and strikes), first base, second base, and third base.
Each game, the four man crew rotates clockwise, so the ump who worked third base yesterday will work second base today.
Major League Baseball (MLB) designates one member of each grouping, generally by seniority, to be the crew chief.
MLB also has the ability to call up umpires from the minor leagues in the case of injury, illness, or other absences.
During most playoff games, six umpires are deployed, with the extra two being stationed down the right- and left-field foul lines.