SGR 21 — Doubly underlined

Render text with a double underline — distinct from single underline (SGR 4).

Byte forms

Every common string-literal form so you can paste-and-search either direction.

\\x1b[\x1b[21m

\\033[\033[21m

\\e[\e[21m

ESC [ESC [ 21 m

hex1b 5b 32 31 6d

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Double underline

Description

Double underline. ECMA-48 §8.3.117 defines SGR 21 as DOUBLY UNDERLINED. Caveat: historically some terminals (older xterm builds, certain Linux console fonts) treated SGR 21 as BOLD OFF instead — a vestige from when bold and double-underline were both intensity-related flags on hardware terminals. Modern xterm, kitty, wezterm, iTerm2, Windows Terminal, ghostty, alacritty all implement the ECMA-48 meaning. If you also need to support legacy terminals, use Kitty's underline-style extension \x1b[4:2m instead — it unambiguously means double underline and is supported by the same modern set plus kitty. Turn off with \x1b[24m (same disabler as single underline).

Spec citation: ECMA-48 §8.3.117 (SGR parameter 21)

Examples

bash

printf '\033[21mdouble underline\033[24m back\033[0m\n'

python

print('\x1b[21mdouble underline\x1b[0m')

fmt.Print("\x1b[21mdouble underline\x1b[0m")

javascript

console.log('\x1b[21mdouble underline\x1b[0m')

printf("\x1b[21mdouble underline\x1b[0m\n");

Terminal support

xterm: yes
Linux console (fbcon): partial
macOS Terminal.app: no
iTerm2: yes
Windows Terminal: yes
cmd.exe / ConPTY: no
kitty: yes
alacritty: yes
WezTerm: yes
Ghostty: yes
GNOME Terminal: partial
Konsole: yes
tmux: yes
GNU screen: yes

xterm	Linux console (fbcon)	macOS Terminal.app	iTerm2	Windows Terminal	cmd.exe / ConPTY	kitty	alacritty	WezTerm	Ghostty	GNOME Terminal	Konsole	tmux	GNU screen
yes	partial	no	yes	yes	no	yes	yes	yes	yes	partial	yes	yes	yes

Related sequences

In the family cookbook

SGR cookbook · 6. SGR colon sub-params — the standards-correct form