Double underline (SGR 21)

Category: sgr · Tags: ECMA-48 Standard · Specification ↗

SGR 21 draws a double underline beneath text. The sequence is ESC [ 21 m. Also accessible via the Kitty sub-parameter syntax as ESC [ 4:2 m. Reset with SGR 24. Support varies — some older terminals treat SGR 21 as bold-off rather than double underline.

Supported by 5 of 6 backends (83%)

Support Matrix

Backend	Version	Support	Notes
Ghostty	1.3.1	✓ yes
Kitty	0.40.0	✓ yes
vt100	0.1.0	✓ yes
WezTerm	0.1.0-fork.5	✓ yes
xterm.js	5.5.0	✓ yes
Alacritty	0.26.0	~ partial	Alacritty supports double underline in app but not in headless mode

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