Common Braille characters are based on a 6-dot cell having two columns of three dots. If the empty cell is counted, 64 unique dot combinations are possible with a six-dot cell. 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 braille page is 11 by 11 inches and typically has a maximum of 40 to 42 braille cells per line and 25 lines.
On-line refreshable Braille displays for computers are available from a number of commercial sources. Dot spacing and height is typically slightly larger than that used for paper braille. Because the mechanical mechanisms used for raising the pins are expensive and fragile, these devices are quite expensive and commonly need frequent service/repair.The most popular models can display a single row of 40 braille cells and cost more than $5000.
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.