The official rules of Major League Baseball require that all players on a team wear matching uniforms, although this rule was not in force in the early days. Originally, teams were primarily distinguished by the colors of their stockings and the success of the Cincinnati Red Stockings popularized the adoption of sock color as the explicit identity of the club. The 1876 Chicago White Stockings actually wore caps of different colors. In 1882, the National League assigned stocking colors to the member clubs: red for Boston, white for Chicago, grey for Buffalo, blue for Worcester, gold for Detroit, green for Troy, and so on. That year, the league also assigned jersey and cap colors, but by player position rather than by club.
Traditionally, when playing at home, teams wore uniforms that were mostly white with trim in team colors and when playing away, they wore uniforms that were mostly gray with trim in team colors. Aside from the obvious need to distinguish one team from the other, conventional wisdom held that it was more difficult to properly launder uniforms while on a road trip, thus the "road grays" helped to hide accumulated soil. This convention continued well after its original premise was nullified by the issuance of multiple uniforms and the growth of the laundromat industry. Starting in the 1970s, with the advent of synthetic fabrics, teams began using more color in their uniforms, notably the Kansas City Athletics in 1963, the San Diego Padres' unusual brown-and-yellow scheme beginning in 1969, and the Houston Astros' rainbow stripes in the mid-1970s. In the late 1970s, the Pittsburgh Pirates began a trend of multiple combinations of differently colored jerseys and trousers and caps (with the options of black, yellow, and white with pin stripes). At one point in the 1970s, the Cleveland Indians had an all-red uniform.
In his comedy routine "Baseball & Football," George Carlin observes that in baseball, as compared to football, the manager is required to wear the same uniform the players do. However, this was actually not true in the early years of the game. Player-managers were common, but non-playing managers whose realm was strictly the dugout often wore business suits, a common occurrence at the time. Retired players who became managers were more likely to continue to wear a baseball uniform (John McGraw, for example), especially if they were also active on the coaching lines; managers often doubled as third-base coach. By the late 1940s, nearly all managers were wearing baseball uniforms. Connie Mack was the last major league manager to wear a suit in the dugout until his retirement in the early 1950s; however, in contrast to the uniform-wearing managers, Mack rarely if ever stepped onto the field during a game; instead he sent uniformed coaches onto the field when a managerial presence outside the dugout was required.
Starting in the 1990s, MLB clubs began heavily marketing licensed goods, such as caps and uniform jerseys to the public and this has resulted in a wide array of uniforms for each team. Now, some teams have not only a basic home uniform and away uniform, but also special "Sunday game" uniforms and uniforms that are worn only during batting practice and uniforms worn on singular events. From time to time, individual MLB teams have held "Turn Back the Clock Day", regularly-scheduled games in which teams donned uniforms in styles their predecessors wore generations earlier (sometimes called "throwback" uniforms), or other antique-style uniforms such as those of Negro League clubs. The Los Angeles Dodgers occasionally use the livery of their original identity as the Brooklyn Dodgers, on special anniversaries or occasions, for example such as in honor of the retirement of Jackie Robinson's uniform number 42 throughout professional baseball (on April 15 - the anniversary of Robinson's MLB debut - entire teams often wear 42). In addition, in 1999, MLB staged "Turn Ahead the Clock Day," in which teams wore futuristic, somewhat strange-looking uniforms, including futuristic or science fiction references, such as the New York Mets being referred to as the "Mercury Mets."
The result is that it is now often difficult to say which uniform is a team's "official" one. For example, from 1999 to 2006 the Cincinnati Reds wore a variety of caps: all red, red crown and black bill, black crown and red bill, and all black, but since 2007, only the all-red (home) and red crown/black bill (away) are used. In contrast from the pre-1970s era, in which there usually was just one home uniform and one road uniform (with certain exceptions, such as Oakland and Pittsburgh's complex combinations), today choices of what combination of uniform elements are worn are now sometimes left up to players. In some cases, aspects of the uniform that are considered official are now rarely worn, such as the New York Mets' blue home cap, with the orange button, which is rarely seen on the field today in favor of an "alternate" black-and-blue cap. The Mets added the orange button on their blue caps in 1995. Through 2010, the New York Yankees, Detroit Tigers and Los Angeles Dodgers, three teams whose uniforms have changed little since the 1930s (only the Tigers, Yankees and Washington Nationals retain the once-common practice of placing the cap insignia on their home whites) were the only teams that didn't wear alternate uniforms. The Dodgers (a blue jersey once in 1999) and Tigers (a navy jersey twice in 1995) had worn alternate uniforms in the past, but as of 2010 did not have one (the St. Louis Cardinals have never worn an alternate jersey, but since 1998 have used a different cap, with a modification of their "birds-on-bat" logo replacing the traditional interlocking StL, for Sunday home games). Typically, home uniforms feature the team’s nickname, while away uniforms feature the name of the team’s home city or state. Currently, the Tampa Bay Rays, Florida Marlins (having removed "Florida" from their road uniforms in 2010 as they intend to become the Miami Marlins when their new stadium opens in 2012), Los Angeles Angels of Anaheim, Philadelphia Phillies, St. Louis Cardinals, Texas Rangers, and Milwaukee Brewers are the only exceptions to this rule, although the Brewers reintroduced "Milwaukee" on their alternate away uniforms in 2010. These teams feature the club nickname on both the home and away uniforms (since 1900, in 165 seasons - including the Athletics' presence in the city from 1901 to 1964 - the full word "Philadelphia" has never appeared on a Major League jersey), except for the Rangers, who feature "Texas" on both their home and road uniforms. From 1973 to 2008, the Baltimore Orioles were part of this group - the omission of the city's name being part of a largely successful effort to attract fans from the Washington, D.C. area - before returning "Baltimore" to the road jerseys in 2009, by which time the their neighbor 38 miles to the south once again a team of its own.
MLB uniforms have featured an MLB on the back collar jerseys since 2000, and MLB logos on the back of caps since 1992. The jerseys that have been worn on the field since 1973, are made out of double knit polyester. Since 2007, MLB caps were no longer made out of wool, but of polyester. The New York Yankees were among the last teams in MLB to wear the caps with the green underbrim in 1993. They didn't switch to the gray underbrim until 1994, when most teams have had a gray underbrim since the late 1980s.
The official rules state that:
- All players on a team must wear identical uniforms during a single game.
- Numbers: All players must wear their uniform numbers on the back of the uniform.Further information: Uniform number (Major League Baseball)
- Undershirt: If the undershirt is exposed then all the players on the team must wear matching ones. Numbers or other devices may be worn on the sleeve of the undershirt (for example, if it is worn with a sleeveless jersey), except that pitchers may not have such devices on their undershirt sleeves.
- The league office might require that each team have a single uniform for all games or requires that each team have a single, white home uniform and a single, non-white away uniform. With the elimination of the separate American League and National League administrations, it is unknown what the effectiveness of this rule now is.
- Sleeve length: The rules allow for minor variation in sleeve length, but they must be "approximately the same length" and the sleeves may not be "ragged, frayed or slit."
- No attachments: Tape or other attachments of non-matching color may not be used on uniforms. Pants may not be attached to the bottom of the shoe in any manner.
- No images of baseballs: No "pattern that imitates or suggests the shape of a baseball" may be used on uniforms. Notably, in apparent violation of this rule, the Toronto Blue Jays, Milwaukee Brewers, Los Angeles Angels of Anaheim, Florida Marlins and Philadelphia Phillies for many years had logos that incorporated the image of a baseball. However, while the Philadelphia cap logo clearly depicted the baseball, the logo worn on the uniform jersey did not feature the image of the stylized stitching indicating the image of a baseball. The Marlins logo, while depicting a baseball, can not easily be mistaken for one, as the team's namesake fish is displayed in front of the ball design. The Toronto Blue Jays had a similar design with a Blue Jay head on the front of the ball, and this logo was even used on the center (and later left-center)of the uniform itself. The classic Brewers "ball and glove" logo (with the team's initials, MB, stylized into the shape of a blue baseball glove surrounding a ball) made a comeback in 2006 on the hats of the Brewers' Sunday home uniforms and is now the Brewers' Friday home uniform. Also, many teams such as the Giants, Nationals, Rangers and the Mariners use uniform logos that clearly depict a baseball, the New York Mets have featured uniforms with a sleeve logo that imitates the appearance of a baseball since the team's inception in 1962, so it may be that the rule is not enforced, at least for caps. (The purpose of this rule is to prevent one team from deceiving the other. The National Football League has a similar rule, which states that no pattern that imitates or suggests the shape of a football).
- No glass buttons or polished metal.
- No commercial advertisements on uniforms. This rule is in variance with other professional sports, such as the Arena Football League and, most prominently, NASCAR, the NHRA, and the IRL in the United States, but especially outside the US (notably soccer), in which it is customary for uniforms to prominently display the logo of a sponsoring company. The commemorative patches worn by the New York Mets during their inaugural season at the Citigroup sponsored Citi Field did not feature the name of the ballpark in adherence to this rule. However, when the New York Yankees and Tampa Bay Devil Rays opened the season in Japan in 2004, an ad for Ricoh was clearly visible on the batters' helmets. When the Oakland Athletics and Boston Red Sox opened the 2008 season in Tokyo, not only did both teams wear batting helmets featuring the Ricoh ad; but also, the Red Sox featured a commercial advertisement for a New England-based business on their jerseys and the A's jerseys featured an advertisement for Pepsi. Exceptions are made for the manufacturers of the pieces of uniform or equipment upon which they are placed (i.e. the hat manufacturer's emblem may be on the hat).
- Names: "A league may provide that the uniforms of its member teams include the names of its players on their backs. Any name other than the last name of the player must be approved by the League President. If adopted, all uniforms for a team must have the names of its players." Again, with the elimination of separate administrations for the American and National leagues, it is unknown what the provenance of this rule is. (Currently, Ichiro Suzuki of the Seattle Mariners is the only player to have his given name rather than his family name displayed on the back of his uniform, having applied for this permission in order to continue being identified as he had been in the Japanese leagues. Vida Blue also used his first name on the back of his uniform when he played for the San Francisco Giants in the mid-1980s). As of 2010, the New York Yankees, Boston Red Sox and San Francisco Giants do not display their players' names on their home uniforms (the Giants did adopt the names in the 1970s, but removed them from the home uniform in 2000); the Yankees also do not display them on their road uniforms. The New York Mets used alternate home uniforms without last names for the 1999 season. The names were returned the next season. The Chicago Cubs did not have names on their home or alternate jerseys for the 2005 and 2006 seasons. The names are now back on both jerseys. The Los Angeles Dodgers did not have names on the back of their home and road jerseys for the 2005 and 2006 seasons. Names returned on both jerseys in 2007.
Another apparent violation of the concept of a "uniform" is that some players on a team will wear the traditional knee-breeches or "knickers" while other teammates are wearing the more-recent ankle-length, closely-cut trousers. Many clubs do this at both major and minor league level, with no apparent objections.
On game days that do not require a special uniform (either by team or MLB request) it is generally (but not always) the starting pitcher for a team that chooses the uniform to be worn for that day's game.