Control characters are characters that do not represent printable characters or symbols, but are used to control the interpretation or display of text¹. They are also used as in-band signaling to cause effects other than the addition of a symbol to the text².

For example, the control character BEL (code 7) is used to ring a terminal bell, the control character CR (code 13) is used to move the cursor to the beginning of the line, and the control character ESC (code 27) is used to introduce a control sequence or an escape sequence¹.

There are different types of control characters, such as printing control characters, data structuring control characters, and transmission control characters². Some control characters are also called format effectors, because they affect the presentation of plain text¹.

Control characters are divided into three parts:

• ASCII control characters: These are the code points from 0 to 31 (hex 00 to 1F) and 127 (hex 7F) in the ASCII standard. They are also called the C0 and C1 control codes. They cover various applications, such as format effectors, transmission controls, device controls, and information separators¹.

• Unicode control characters: These are the code points from 128 to 159 (hex 80 to 9F) and from 240 to 255 (hex F0 to FF) in the Unicode standard. They are also called the C2 and C3 control codes. They include some additional format effectors, such as bidirectional text controls and variation selectors³.

• Other non-printing characters: These are the code points that have the general category Cf (format) in the Unicode standard. They are not technically control codes, but they are used for format effectors introduced and defined by Unicode itself. They include zero-width spaces, soft hyphens, byte order marks, and joiners³.