Eight-dot braille cell

We can form an eight-dot braille cell by adding a fourth row of dot positions below the usual six-dot braille cell. This eight-dot braille cell has eight dot positions arranged in two columns and four rows. We traditionally number these dot positions from one to eight in such a way as to retain the usual numbering for the dot positions for the upper six dot positions:

1. First or top row, left-hand column
2. Second row, left-hand column
3. Third row, left-hand column
4. First or top row, right-hand column
5. Second row, right-hand column
6. Third row, right-hand column
7. Fourth or bottom row, left-hand column
8. Fourth or bottom row, right-hand column

A dot can be either present or absent in each dot position. A cell with eight dot positions gives 256 different characters, because 2 raised to the power 8 is 256.

We can represent a braille character in words by listing the numbers of the dot positions where dots are present. For example, we can refer to a braille character that has dots in dot positions 1 and 5 as "dots 1 5" or "dots 1-5" or, slightly more concisely, as "dots 15", all of which we read as "dots one five".