The term is often shortened to the colloquial form ump. They are also addressed as blue at lower levels due to the common color of the uniform worn by umpires. In professional baseball, the blue is seldom used by players or managers. In a game officiated by two or more umpires, the umpire in chief is the umpire who is in charge of the entire game. This umpire calls balls and strikes, calls fair balls, foul balls short of first/third base, to avoid injury, the home plate umpire wears the same mask the catcher wears. If another umpire leaves the infield to cover a potential play in foul ground or in the outfield, in the event that an umpire is injured and only three remain, the second base position will generally be left vacant. In nearly all levels of organized baseball, including the majors, an umpiring crew rotates so that each umpire in the works each position, including plate umpire. In the earliest days of baseball, however, many senior umpires always worked the plate, Klem did so for the first 16 years of his career. On the Major League level, an umpiring crew generally rotates positions clockwise each game, for example, the plate umpire in one game would umpire third base in the next. Other umpires are called base umpires and are stationed near the bases. When two umpires are used, the umpire is simply the base umpire. This umpire will make most calls concerning runners on the bases and nearby plays, to indicate a full swing, he will clench his fist. When four umpires are used, each umpire is named for the base at which he is stationed, sometimes a league will provide six umpires, the extra two are stationed along the outfield foul lines are called the left-field and right-field umpires. Outfield umpires are used in events, such as the Major League Baseball All-Star Game. For Major League Baseball, all levels use six umpires, while at lower levels. Rulings on catches of batted balls are made by the umpire closest to the play. The term umpire-in-chief is not to be confused with the crew chief, at the major-league and high minor-league levels, the crew chief acts as a liaison between the league office and the crew and has a supervisory role over other members of the crew. For example, on the Major League level, The Crew Chief shall coordinate, an umpires judgment call used to be final, unless the umpire making the call chose to ask his partner for help and then decided to reverse it after the discussion
Home plate umpire Malachi Moore reviews the lineup cards from both teams before a 2016 minor league baseball game
A second base umpire (Mike Reilly) at an MLB game in 2008.
Umpire Tom Connolly shown with the outside chest protector worn by AL home plate umpires for much of the 20th century