This is taken from the current Braille article, from the section on braille writing.
Braille writing may be produced using a slate and stylus in which each dot is created from the back of the page, writing in mirror image, by hand, or it may be produced on a Braille typewriter or Perkins Brailler, or produced by a Braille embosser attached to a computer. It may also be rendered using a refreshable Braille display.
Braille has been extended to an 8-dot code, particularly for use with Braille embossers and refreshable Braille displays. In 8-dot Braille the additional dots are added at the bottom of the cell, giving a matrix 4 dots high by 2 dots wide. The additional dots are given the numbers 7 (for the lower-left dot) and 8 (for the lower-right dot). Eight-dot Braille has the advantages that the case of an individual letter is directly coded in the cell containing the letter and that all the printable ASCII characters can be represented in a single cell. All 256 (28) possible combination of 8 dots are encoded by the Unicode standard. Braille with six dots is frequently stored as Braille ASCII.
The first ten letters of the alphabet and the numbers 1 through 0 are formed using only the top four dots (1, 2, 4, and 5). Adding dot 3 forms the next ten letters, and adding dot 6 forms the last five letters (except w) and the words and, for, of, the, and with. Omitting dot 3 forms the letters U-Z and the five word symbols form nine digraphs (ch, gh, sh, th, wh, ed, er, ou, and ow) and the letter w.
Letters and numbers
* The question mark is represented by dots 2-3-6—the same as the opening quotation mark. Therefore the placement of the dots—before a word or after a word—will determine which symbol it is.
* Opening and closing parentheses are shown with the same symbol. Therefore, the placement context will determine whether the parentheses is opening or closing.
Grade 2 Braille contractions
Braille also includes a number of whole word contractions, for example the word Braille becomes a three cell word brl.
Unicode rendering table
The Unicode standard encodes 8-dot Braille glyphs according to their binary appearance, rather than following the alphabetic order of any particular convention. Unicode defines the "Braille Patterns" character block in the hex codepoint range from 2800 to 28FF.