The following are not the official uniform decoration rules but general summaries. To view the actual rules as set by the MLB, see Rules 1.11 and 1.17 under the heading "Objectives of the Game" at the Major League Baseball Website.
Uniforms must be identical
All players of the same team must wear identical uniforms.
Numbers must be worn on the back, and may be no smaller than six inches.
Any exposed parts of undershirts must be the same color for every player on the team.
All players except the pitcher may have numbers, letters, or their insignia attached to the sleeve of the undershirt. Any teammate whose uniform does not comply with these rules will not be allowed to play in the game.
Should a team have one or two uniforms?
Leagues may decide if teams should wear the same certain uniform for every game, or if teams should have two different uniforms; one for home games and one for road games.
Sleeves for each player on a team must be hemmed, not ragged, cut off or slit, and be approximately the same length.
Attachments to uniforms
Any materials attached to a uniform must be the same color as the uniform.
No baseball patterns
No part of the uniform may include any pattern that imitates or is the same shape or likeness as a baseball.
No glass or metal attached to uniform
Glass buttons or polished metal may not be used on a uniform.
No spikes or cleats on shoes
Players may not attach anything to the heels or toes of shoes except for shoe or toe plates.
Pointed spikes similar to golfing or track spikes must not be used.
No commercial patches or designs may be used on any part of the uniform.
Use of players' names at the discretion of the League
Leagues may decide if players' names will be displayed on the backs of uniforms and all uniforms must follow suit. Names other than the last names of players must be approved by the League president.
Rule 1.17 says that bases, pitcher's plate, baseball, bats, uniforms, and other equipment must not display any unnecessary advertising of the product, and any such brand logos or symbols must be in "good taste" with respect to its size and content. This rule applies to professional leagues only. The ruling also notes: "Manufacturers who plan innovative changes in baseball equipment for professional baseball leagues should submit same to the Official Playing Rules Committee prior to production."