What are ASCII control characters?

Control characters are the non-printing codes 0-31 plus DEL (127) in ASCII. Instead of displaying a glyph, they signal actions such as newline, tab, backspace or interrupt. They form the C0 control set.

What is caret notation?

Caret notation writes a control character as ^ followed by a letter, mapping Ctrl+key to a code. For example ^C is ETX (3), produced by pressing Ctrl+C, and ^? is DEL (127). The letter equals the code plus 64.

What is the difference between LF and CR?

Line Feed (LF, 10, \n) moves down one line, while Carriage Return (CR, 13, \r) returns the cursor to the start of the line. Unix uses LF alone for newlines, Windows uses CR+LF, and classic Mac used CR alone.

Why does Ctrl+C interrupt a program?

Ctrl+C sends the ETX control character (3), which terminals translate into a SIGINT signal. SIGINT asks the foreground program to stop, which is why it cancels running commands.

Are control characters still used today?

Yes. NUL, LF, CR, HT (tab) and ESC are used constantly in text, terminals and protocols. ESC in particular begins every ANSI escape sequence. Others like the file/group/record/unit separators survive in data formats and legacy systems.

What is the Control Characters Reference?

Interactive reference for ASCII control characters 0-31 and DEL (127), including caret notation, C escape equivalents, abbreviations, full names and what each one does in modern systems. It runs free in your browser on Gera Tools, with nothing uploaded.

Control Characters Reference

Name: Control Characters Reference
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

The non-printing side of ASCII

The first 32 ASCII codes (0–31), plus DEL at 127, are control characters: they do not display a glyph but instead instruct the device to perform an action such as advancing a line, sounding a bell, or interrupting a program. This reference lists every C0 control character with its decimal and hex value, caret notation, C escape sequence where one exists, abbreviation, full name, and a note on its use.

Where control characters come from

Control characters were designed in the 1960s for teletype machines and early terminals, where communication channels needed in-band signalling to control paper advancing, cursor movement, and device state. The ASCII standard (ANSI X3.4-1963, revised 1967 and 1968) codified the C0 set: 32 control codes plus DEL, covering everything from flow control (XON/XOFF) to media navigation (FS, GS, RS, US) to character encoding state (SO/SI for switching character sets).

Most of the original use cases for control characters are obsolete — no one is advancing paper rolls anymore. But many survive in daily use:

Character	Code	Still used for
NUL	0	String terminator in C, null byte in binary protocols
BEL	7	Terminal alerts; still triggers a beep or visual bell
BS	8	Backspace in text input
HT	9	Tab character in text, source code, TSV data
LF	10	Line ending on Unix/macOS/Linux
CR	13	Part of CRLF line ending on Windows; line ending in HTTP headers
ESC	27	Start of ANSI/VT100 terminal escape sequences
DEL	127	Forward-delete key in many terminals

How caret notation maps to keys

Caret notation represents each control character as ^ followed by an uppercase letter. The rule is simple: the caret letter’s ASCII code is the control character’s code plus 64. So:

^A = code 1   (Ctrl+A, SOH — Start of Heading)
^C = code 3   (Ctrl+C, ETX — signals terminal interrupt / SIGINT)
^D = code 4   (Ctrl+D, EOT — signals end-of-file in many Unix programs)
^G = code 7   (Ctrl+G, BEL — audible bell)
^H = code 8   (Ctrl+H, BS  — backspace)
^I = code 9   (Ctrl+I, HT  — horizontal tab)
^J = code 10  (Ctrl+J, LF  — newline, same as Enter in many contexts)
^M = code 13  (Ctrl+M, CR  — carriage return)
^[ = code 27  (Ctrl+[, ESC — same key as Escape)
^? = code 127 (DEL   — this is the special case; DEL = 64+63 = ^_ is unused)

The ^? for DEL is an exception: DEL (127) does not follow the +64 formula; it is assigned ^? by convention because 127 mod 64 = 63 = ?.

LF versus CR versus CRLF — the line ending wars

Line endings are the most common place developers encounter control characters:

Unix / macOS / Linux: LF only (\n, code 10). This is the POSIX standard.
Windows: CRLF (\r\n, codes 13 then 10). Inherited from teletype convention.
Classic Mac OS (before OS X): CR only (\r, code 13). Changed to LF with OS X.
HTTP headers: CRLF is required by the HTTP specification, regardless of OS.
Network protocols (SMTP, FTP): CRLF by RFC convention.

The mismatch between Unix and Windows line endings causes frequent problems with Git, text editors, shell scripts, and diff tools. Git’s core.autocrlf setting and the .gitattributes file exist specifically to handle this.

ESC and ANSI escape sequences

ESC (27, \x1b, \033 in octal) is the gateway to all ANSI/VT100 terminal control sequences — cursor movement, colour output, screen clearing, and more. An ANSI escape sequence begins with ESC and is followed by [ (the CSI introducer) and a sequence of parameters ending in a letter:

ESC[0m    — reset all formatting
ESC[1m    — bold
ESC[31m   — red foreground
ESC[2J    — clear screen
ESC[H     — move cursor to home position

Understanding that ESC starts these sequences explains why pressing Escape in a terminal sometimes causes a brief delay: the terminal is waiting to see whether ESC is the start of a sequence or a standalone key press.

How the reference is searched

The search matches any of the fields at once: decimal code, hex code, abbreviation (like LF, ETX), full name (“Line Feed”), C escape sequence (\n), or caret notation (^J). This makes it easy to look up a character whether you encountered it as a decimal in a hex dump, a caret notation in a keystroke description, or a backslash escape in source code.