The cell dot-3456 (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.
The primary meaning of each of the 64 six-dot cells of (grade 2) SEB is tabulated for the reader's interest. Anything that can be represented by grade 2 SEB can also be written correctly but less efficiently using a subset of these symbols consisting of the letters, numbers, and punctuation marks. This subset comprises the symbol set of grade 1, or uncontracted, Braille.
Standard English Braille has a few ambiguities. Even grade 1 Braille uses some symbols to mean different things depending on context. In addition there are no plus, times, divide, equals sign or many other common symbols used in math and technical expressions. Consequently special Braille codes have been developed specifically for mathematics and other technical subjects. 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. This number definition is possible because the letters a-j are all upper cells. 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 number 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 specific subjects. Most are very similar to SEB or to the math code used in a given country. However the braille code for music bears little resemblance to any other braille code. Codes intended for use with refreshable computer braille displays and computer program listings also differ considerably from other codes. The American computer code uses Nemeth dropped numbers, and the British computer code uses yet another convention for numbers.
Readers are referred to an excellent discussion of the Nemeth math code and other issues of accessibility to mathematics by Susan Osterhaus of the Texas School for the Blind and Visually Impaired.