How do I draw a curly / wavy underline (the spell-check style)?

Use the kitty styled-underline extension: \x1b[4:3m for wavy, \x1b[4:2m double, \x1b[4:4m dotted, \x1b[4:5m dashed. Combine with a coloured underline via \x1b[58;5;Nm (256-colour) or \x1b[58;2;R;G;Bm (truecolor). Supported on kitty, alacritty, wezterm, ghostty, iTerm2, and recent Windows Terminal — fall back to plain \x1b[4m elsewhere.

What's the difference between \x1b[24m and \x1b[0m when I want to stop underlining?

\x1b[24m is the surgical underline-off: it removes the line and leaves colour, weight, italic intact. \x1b[0m resets *everything*, so any colour or bold you set alongside the underline goes too. Prefer \x1b[24m inside styled passages; reserve \x1b[0m for the end of a fully-styled chunk.

SGR 4 — Underline

Render following text with an underline.

Byte forms

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

\\x1b[\x1b[4m

\\033[\033[4m

\\e[\e[4m

ESC [ESC [ 4 m

hex1b 5b 34 6d

Live preview

Rendered in your browser via the same tokeniser the decoder uses — no terminal needed.

Underlined text

Description

Single underline. The most universally rendered SGR attribute after color and reverse — every terminal in the support matrix above honors at least the basic single line, including the Linux console (which uses a sub-baseline dotted line on framebuffer mode) and ConPTY (partial, only when the host terminal handles the escape itself). Glyphs are underlined via a 1-pixel rule at descender height; visually it stacks fine with sgr-bold and color attributes.

Kitty introduced styled underlines as sub-parameter extensions to SGR 4, since adopted by alacritty, wezterm, ghostty, iTerm2 and recent Windows Terminal: \x1b[4:1m is an explicit single underline, \x1b[4:2m is a double rule, \x1b[4:3m is a curly / wavy line (the spell-check style — useful for highlighting errors), \x1b[4:4m is dotted, \x1b[4:5m is dashed. Pair with colored underlines via \x1b[58;5;Nm (256-color) or \x1b[58;2;R;G;Bm (truecolor), reset with \x1b[59m. Note the sub-parameter form uses : not ;; older terminals that don't understand sub-params will (per ECMA-48 quirks) treat 4:3 as a parse error and may render nothing — feature-detect via $TERM_PROGRAM / $COLORTERM, fall back to \x1b[4m plain underline. The double-rule \x1b[21m (ECMA-48's original 'doubly underlined') is recognized by xterm and most modern terminals; a few legacy terminals overload SGR 21 as 'bold off', so \x1b[4:2m is the safer way to spell 'double' on modern stacks.

Disable underline with \x1b[24m — surgical, leaves color and weight intact. The surgical close-pair is \x1b[4m…\x1b[24m. A common related pattern is clickable text in compatible terminals: combine osc-hyperlink (\x1b]8;;https://…\x1b\\link text\x1b]8;;\x1b\\) with \x1b[4;34m for the familiar blue-underlined-link look; iTerm2, ghostty, wezterm and recent gnome-terminal will render it as a true clickable link. See osc-hyperlink for the protocol details.

Spec citation: ECMA-48 §8.3.117 (SGR parameter 4); kitty underline extensions

Examples

bash

printf '\033[4munderline\033[24m back\033[0m\n'

python

print('\x1b[4munderline\x1b[0m')

fmt.Print("\x1b[4munderline\x1b[0m")

javascript

console.log('\x1b[4munderline\x1b[0m')

printf("\x1b[4munderline\x1b[0m\n");

Used in

Real-world tools that emit this sequence — anchors the bytes to commands you've already used.

man pages (section headings)
git diff --color-wordsintra-line emphasis on the changed token
modern terminals on OSC 8 hyperlinksiTerm2, kitty, WezTerm, Windows Terminal auto-underline clickable links
less search match

Frequently asked

Short answers to the questions developers actually search for this sequence.

How do I draw a curly / wavy underline (the spell-check style)?: Use the kitty styled-underline extension: \x1b[4:3m for wavy, \x1b[4:2m double, \x1b[4:4m dotted, \x1b[4:5m dashed. Combine with a coloured underline via \x1b[58;5;Nm (256-colour) or \x1b[58;2;R;G;Bm (truecolor). Supported on kitty, alacritty, wezterm, ghostty, iTerm2, and recent Windows Terminal — fall back to plain \x1b[4m elsewhere.
What's the difference between \x1b[24m and \x1b[0m when I want to stop underlining?: \x1b[24m is the surgical underline-off: it removes the line and leaves colour, weight, italic intact. \x1b[0m resets *everything*, so any colour or bold you set alongside the underline goes too. Prefer \x1b[24m inside styled passages; reserve \x1b[0m for the end of a fully-styled chunk.

Common typos / look-alikes

Wrong forms developers commonly paste from Stack Overflow that look right but aren't — with the canonical correction inline.

\x1b[4mtext\x1b[0m (cargo-culting full reset to close underline)\x1b[4mtext\x1b[24m
Mirror of the sgr-bold mistake — using \x1b[0m to close an underline span wipes every other SGR attribute in scope. If the underline is nested inside coloured / bold / italic text, the 0m resets all of them. Close underline with \x1b[24m (its dedicated off-code), and reserve \x1b[0m for the boundary where you genuinely want to reset everything.
\x1b[4:1m (ITU T.416 colon sub-parameter) on macOS Terminal / fbcon\x1b[4m for straight underline; gate \x1b[4:3m (curly) on terminal support
Extended underline styles (4:1 straight, 4:2 double, 4:3 curly, 4:4 dotted, 4:5 dashed) use the ITU T.416 colon : sub-parameter separator that only kitty / WezTerm / iTerm2 ≥ 3.5 / Alacritty / xterm ≥ patch 357 / Windows Terminal ≥ 1.21 support. macOS Terminal.app, gnome-terminal < 3.34, and the Linux fbcon parse 4:3m as 'SGR 4, then :3m as literal text' — printing the literal :3m after a normal underline. Probe via XTVERSION / DA3 before emitting curly underline.
\x1b[58;5;Nm (set underline colour) ignored on most terminals\x1b[4m\x1b[58;5;Nm…\x1b[59m\x1b[24m; gate on kitty / WezTerm / VTE ≥ 0.52
SGR 58 (set underline colour, kitty extension also in VTE) is a coloured-underline subset of T.416 with the same support story — kitty, WezTerm, gnome-terminal (VTE) ≥ 0.52, iTerm2 ≥ 3.5, Alacritty pick it up; older terminals print the literal bytes. Worse, the corresponding *reset* is SGR 59 (default underline colour) — not 0 — and forgetting it leaves a red wavy underline persisting across colour resets, because most terminals don't tie underline colour to the SGR 0 wipe.

Terminal support

xterm: yes
Linux console (fbcon): yes
macOS Terminal.app: yes
iTerm2: yes
Windows Terminal: yes
cmd.exe / ConPTY: partial
kitty: yes
alacritty: yes
WezTerm: yes
Ghostty: yes
GNOME Terminal: yes
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	yes	yes	yes	yes	partial	yes	yes	yes	yes	yes	yes	yes	yes

Related sequences

In the family cookbook

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