What is the Braille system?

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A basic Braille cell
According to Dictionary.com, Braille is "a system of writing or printing, devised by Louis Braille for use by the blind, in which combinations of tangible dots or points are used to represent letters, characters, numbers, etc., that are read by touch, or to write or transliterate in Braille characters."

Essentially, Braille is a system used for reading and writing by the blind.  Each "letter" in the Braille system is a combination of dots in a "Braille cell."  There are 6 dots in the cell, and all Braille characters follow the layout of the two-by-three rectangle.  Braille is read by running a finger along a line, mentally reading or translating the embossed bumps.  Usually, it is written with a slate and stylus.  New technology has lead to other ways to write Braille, and new uses for it (see "Methods for Writing Braille" and "Uses for Braille.")