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In American English, bleachers or stands are raised, tiered rows of seats found at sports fields and other spectator events. Stairways access the horizontal rows of seats, often with every other step gaining access to a row of seats.
Seats range from simple plank seats to more elaborate seats with backrests. Many bleachers are open to the ground below so that there are only the planks to sit and walk on. Some bleachers have vertical panels beneath the seats, either partially or completely blocking the way to the ground.
Name origins 
A key feature of bleachers is that they are typically uncovered and unprotected from the sun; thus the wooden seats were "bleached by the sun." The term "bleachers" used in this sense can be traced back to at least 1889. The Dickson Baseball Dictionary states that the open seating area was called the "bleaching boards", as early as 1877. Dickson lists as a secondary definition the fans sitting in them. By the early 1900s, the term "bleachers" was being used for both the seating area and its occupants.
In modern usage the term "bleachers" almost always refers to just the seating area, and those sitting there may be called "bleacher fans", or "bleacherites". Terms such as Chicago's "bleacher bums", or Yankee Stadium's Bleacher Creatures are also used.
Bleacher structures vary depending on the location, but most outdoor modern bleachers have either an aluminium tube or steel angle understructure (known as frame-type bleachers) or steel I-beams (known as an I-beam bleacher). Most smaller bleachers are frame-type bleachers and most larger bleachers are I-Beam bleachers. A bleacher is known as a grandstand when it contains VIP seats. Bleachers range in size from small, modular, aluminum stands that can be moved around soccer or hockey fields to large permanent structures that flank each side of an American football field. Some bleachers have locker rooms underneath them. In indoor gyms, bleachers can be built in so that they slide on a track or on wheels and fold in an accordion-like, stacking manner. These type of bleachers are known as telescoping bleachers.
In baseball stadiums, the bleachers are usually located beyond the outfield fences. However, center-field bleachers are located in the line of sight of the batter, and the presence of fans makes it difficult for the batter to pick out the ball. As a result, most stadiums have vacant areas or black backgrounds where the seats would be. This is known as either the "Backdrop" or the Batter's eye. The old Yankee Stadium featured black-painted vacant bleachers, nicknamed the black by baseball fans.
Though many stadiums offer only bleacher seating, in those that offer both seats and bleachers, the bleachers are typically in less desirable locations and/or have lower ticket prices, giving the term "bleachers" a connotation of lower-class seating.
The popularity of American football has made seating on outdoor and indoor football fields a neccesity. Professional, College, High School, and even Middle Schools have bleacher systems setup to accommodate their fans. They vary in size from 10 feet wide that seat 25 all the way to full stadiums that seat thousands and wrap around the entire field. Its not uncommon to see football bleachers that rise hundreds of feet into the air. Football bleachers are commonly made from concrete or aluminum with concrete footings or superstructure underneath.
Cultural impact 
The term "under the bleachers" is imbued with cultural meaning from the post-war era of American high school football stars and cheerleaders. In the sexually conservative society of post-war America, some students would find places like the bleachers at the football field, or a secluded parking lot, to interact socially and sexually with their peers. The "bleachers" have been given cultural connotations of the innocence of high school and youth. The British equivalent is "behind the bike sheds".
- 'Bleacher' , Online Etymology Dictionary