Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Many Unicode control characters are used to control the interpretation or display of text, but these characters themselves have no visual or spatial representation. For example, the null character is used in C-programming application environments to indicate the end of a string of characters. In this way, these programs only require a single starting memory address for a string, since the string ends once the program reads the null character. The control characters U+0000-U+001F and U+007F come from ASCII. Additionally, U+0080-U+009F were used in conjunction with ISO 8859 character sets. They are specified in ISO 6429 and often referred to as C0 and C1 control codes respectively.