Rule 1.12

Rule 1.12

The catcher may wear a leather mitt not more than thirty-eight inches in circumference, nor more than fifteen and one-half inches from top to bottom. Such limits shall include all lacing and any leather band or facing attached to the outer edge of the mitt. The space between the thumb section and the finger section of the mitt shall not exceed six inches at the top of the mitt and four inches at the base of the thumb crotch. The web shall measure not more than seven inches across the top or more than six inches from its top to the base of the thumb crotch. The web may be either a lacing or lacing through leather tunnels, or a center piece of leather which may be an extension of the palm, connected to the mitt with lacing and constructed so that it will not exceed any of the above mentioned measurements.

DISCUSSION
"The catcher may not substitute a fielder's glove or a first baseman's mitt for a catcher's mitt during the progress of the game or on any individual play." [MLBUM]
"The catcher may not wear a first baseman's mitt or any glove during play." [WRIM]

When catching knuckleballer Tim Wakefield, Red Sox catchers Victor Martinez and Jarrod Saltalamacchia used a first baseman's mitt.

 A catcher’s mitt may have a vinyl fluorescent piece attached. [PBUC]; {MLBUM]

This mitt was introduced in 1975 by Al Campanis, former general manager of the Dodgers and patented. Dodger’s catcher Steve Yeager used one in the 1970’s, and they are still available today.

HISTORY
In 1960 Baltimore Orioles catcher Clint Courtney used a catcher's mitt with a 45-inch circumference to catch knuckleball specialist Hoyt Wilhelm. The photo below shows former MLB catcher Ray Schalk with his 1912 mitt and Courtney's mitt. In 1965 the maximum size of a catcher's mitt was set at 38 inches.