Control characters are special characters that do not render visually but instead direct how text is processed or displayed. The first 32 characters of ASCII (U+0000 through U+001F) and DEL (U+007F) are classified as control characters. Unicode extends this range with additional control characters in U+0080 through U+009F.

Only a handful of control characters see everyday use. LF (Line Feed, U+000A) represents a newline and is the standard line ending on Unix-based systems. CR (Carriage Return, U+000D) moves the cursor to the beginning of the line; Windows uses the two-character sequence CR+LF for line breaks. HT (Horizontal Tab, U+0009) is the tab character used for column alignment. NUL (U+0000) is the null character, used in C to mark the end of a string.

Control characters are a common source of confusion in character counting. When you press Enter in a text editor, the display simply moves to a new line, but internally either LF (1 byte) or CR+LF (2 bytes) is inserted. A text file created on Windows will have a higher character count than the same content created on Unix, because each line break contributes two characters (CR+LF) instead of one (LF).

In the web context, HTML collapses consecutive whitespace characters (spaces, tabs, newlines) into a single space for display. This means the character count in HTML source code does not match the character count of the rendered text. The <pre> element or CSS white-space: pre preserves whitespace including control characters exactly as written. Character encoding references on Amazon cover these distinctions thoroughly.

From a security perspective, control characters can be exploited in injection attacks. HTTP header injection inserts CR+LF into HTTP headers, and log injection inserts newlines to forge log entries. Sanitizing user input by stripping control characters is a fundamental security practice.

Unicode also defines a category called "format characters" that are distinct from control characters. Zero-width space (U+200B), zero-width joiner (U+200D), and bidirectional control characters (U+200E, U+200F) fall into this group. Like control characters, they are invisible, but they influence how text is displayed, so their treatment in character counting must be clearly defined.