A trivial way to secure your information is to keep copies of it.

A somewhat more advanced way is to include control characters in your information. Suppose your information is a string of zero's and one's, then you may add at each 8-th position a control character equal to 0 or 1 such that the sum of the control character and the seven preceding ones are even. So,

110110011010001110011

is transformed into

110110001101000111100111.

A mistake can be detected, but not corrected.

ISBN is the international standard book number. It is a code that each officially published book obtains. This code consists of 9 digits followed by a 10-th symbol from {0,1, ..., 9, x}. The first 9 digits give information on the book, like the year and place it is published. The last symbol is a check symbol.

If the ISBN of a book is

a₁a₂ ··· a₉b,

then the following relation is satisfied.

a₁ + 2a₂ ··· + 9a₉ = b (mod 11)

Here 10 is represented by the symbol x.

If there is a mistake of +1 or -1 in the n-th symbol of a₁a₂ ··· a₉b, then

a₁ + 2a₂ ··· + 9a₉ + 10b (mod 11)

equals +n or -n, respectively. This makes it possible to detect and correct an error.