From BR Bullpen
A Cap, also called a ballcap or baseball cap is the most prominent part of a baseball uniform. The cap is a type of hat composed by a circular head cover made of a rounded top of soft material, lined with a brim or bill designed to protect a player's eyes from the sun. The caps worn by professional baseball players are usually fitted; cheaper models include either an elastic band or a plastic hinge with notches that can be adapted to a range of head-sizes. These last two styles leave an open semi-circle in the back, which is sometimes used to fit a ponytail.
The cap is usually the best-known symbol of a team, much like a football helmet, and copies are liberally worn by its supporters, either at the ballpark or on the streets - unlike the football helmet. Its popularity as a fashion item waxes and wanes (it was practically unknown for anyone older than a teenager to wear one in the 1950s, and in the late 1990s they were ubiquitous, although often worn backwards or sideways, after being popularized by rappers and other stars).
The design of the baseball cap has been largely unchanged since the beginning of the 20th Century. To celebrate the US Bicentennial in 1976, the Pittsburgh Pirates and St. Louis Cardinals innovated by adopting a retro-style cap that was cylindrical in shape and lined with circular bands; the Pirates wore that style for a decade, but it was dropped by the Cardinals after one season and not picked up by any other team.
Traditionally, the cap was all of one color, which was the team's principal color and also was used for the stirrups, warm-up jacket and long-sleeved sweater worn under the uniform jacket. Beginning in the 1960s, it was more common to see two-toned caps, with the bill and crown being two different colors. In 1969, the Montreal Expos went one better, by using three different colors for the crown (red for the two sides, blue for the back and white for the front). That style was adopted by a number of other teams in the 1970s, then went out of fashion by the end of the following decade.
The cap has traditionally been graced on its front by a letter or symbol representing the team. This was at first the initial of the city represented (i.e. B for Boston, D for Detroit or an interlocked N and Y for New York), but over the years this has become more diversified, with teams putting a logo or even a mascot such as Chief Wahoo on the front. Caps are also available in a variety of colors that may or may not be worn by the team on the field (e.g. the pink version of the Boston Red Sox cap which became particularly popular in the 2000s, or caps in camouflage colors), or in alternative designs which substitute a non-traditional symbol for the city letter.
Before the 2014 season, Major League Baseball approved the use of padded caps by pitchers, to protect them against line drives hit through the pitcher's mound, after a number of injuries in previous years, most prominent that of Brandon McCarthy. The proposed caps are a half-inch thicker in the front, and one inch thicker on the sides, while remaining relatively light (in contrast to a batting helmet. While the new style of cap was expected to be adopted in many youth leagues, it was not clear whether any professional pitcher would use them, as their bulkier shape would require a period of adaptation. The new style was also not particularly flattering, giving the wearer a "mushroom-head" look, an important downside in the fashion-conscious world of professional baseball. Alex Torres of the San Diego Padres became the first player to wear the new headgear in a game, on June 22nd.