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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

BackendVersionSupportNotes
Ghostty1.3.1✓ yes
Kitty0.40.0✓ yes
vt1000.1.0✓ yes
WezTerm0.1.0-fork.5✓ yes
xterm.js5.5.0✓ yes
Alacritty0.26.0~ partialAlacritty supports double underline in app but not in headless mode

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