Common Braille characters are based on a 6-dot cell having two columns of three dots. Dot height is approximately 0.02 inches (0.5 mm); the horizontal and vertical spacing between dot centers within a cell is approximately 0.1 in (2.5 mm); the blank space between dots on adjacent cells is approximately 0.15 in (3.75 mm) horizontally and 0.2 in (5.0 mm) vertically. A standard US braille page is 11.5 by 11 inches and typically has a maximum of 40 to 42 braille cells per line and 25 lines.
A six dot cell permits exactly 64 distinct dot patterns if the blank cell is counted. In Standard English Braille (SEB) the cells with no dots in the left column have no meaning individually. They are always used as indicators in combination with another cell, usually with the indicator cell first. All other cells have a meaning, and most have several meanings depending on context.
The cell with dots at the lower left and in the entire right column is a special indicator cell for numbers. A letter a following this sign is interpreted as the number 1, b as 2, c as 3, d as 4, e as 5, f as 6, g as 7, h as 8, i as 9, and j as 0. A multiple digit number requires only a single number indicator at the start.
Commonly the dot position at the upper left of the Braille cell is called dot 1, with dots 2, then 3 below it. Dot 4 is on the upper right with dots 5, then 6 below it. The primary usage of each cell is tabulated for the reader's interest.
Standard English Braille has no plus, times, divide, equals sign or many other common symbols used in math equations. Consequently special codes have been developed specifically for mathematics. In North America, the code developed by Dr. Abraham Nemeth is the officially accepted math code.
Letters in the Nemeth code are those of standard Braille, but nearly every other cell has a different meaning than in SEB. Nemeth numbers for the digits 1-9, 0 are the letters a-i, j except that they are dropped one row. In SEB most of these dropped cells are punctuation marks, so a blind child learning math must learn to interpret dropped cells as punctuation marks when reading text and as numbers when reading math.
The official maths code used in England bears little resemblance to the Nemeth code. It uses the numbr sign indicator for numbers in some instances and Nemeth-style dropped numbers in others.It is also a very different code from SEB that requires people to interpret cells differently when reading math than when reading text.
There are a number of other specialized official codes. For example there are special codes for use with computer on-line refreshable braille displays and for computer programs listed in braille. The US and British computer codes have significant differences from both Standard English Braille and the math codes used in either country.
8-dot braille is being used for some special purposes, largely with computer on-line refreshable braille displays. Many braille printers can also print 8-dot braille. 8-dot braille cells have two columns of four dots. Such cells have 256 unique patterns if the blank cell is counted. Many computer braille displays indicate capital letters with a dot on the lower left (denoted dot 7). Otherwise there is little consistency in the use of the lower two dots. There are a number of official and semi-official 8-dot braille codes, largely in Europe. However, little literature has been reproduced in any 8-dot code.
To our knowledge, little more than anecdotal evidence exists for the ultimate usefulness of 8-dot braille. It is clear that few skilled 6-dot Braille readers could quickly and comfortably begin reading 8-dot code. Most people who have participated in limited testing of the GS8 code report that they must adjust their finger position or other reading styles. Not surprisingly, many testers did not like 8-dot code. However some expressed the opinion that if there is good reason for using an 8-dot code (such as convenience in representing technical materials), then motivated readers could learn and use 8-dot Braille. That opinion has not yet been tested.