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Double underline (SGR 21)
ESC [ 21 mSGR 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.How this is tested: Send
\x1b[21mX, verify underline === "double". Tests whether SGR 21 is interpreted as double underline rather than bold-off.Analysis2026-03-26
Supported by all 8 tested terminals — universal adoption. Part of the Modern TUI baseline.
Supported by 11 of 13 backends (85%)
Terminal Applications
| Terminal | Version | Support | Notes |
|---|---|---|---|
| iTerm2 | 3.6.9 | ✓ yes | |
| Ghostty | 1.3.1 | ✓ yes | |
| VS Code | 1.112.0 | ✓ yes | |
| Warp | 0.2026.03.18.08.24.03 | ✓ yes | |
| Cursor | 2.6.21 | ✓ yes | |
| tmux | 3.6a | ✓ yes | |
| Kitty | 0.46.2 | ✓ yes | |
| Terminal.app | 2.15 | ✓ yes | |
| GNU Screen | 5.0.1 | ✓ yes |
Headless Backends
Parser correctness only — a ✓ means the parser accepts the sequence.
| Backend | Version | Support | Notes |
|---|---|---|---|
| vterm | 0.2.0 | ✓ yes | |
| WezTerm | 0.1.0-fork.5 | ✓ yes | |
| Alacritty | 0.26.0 | ~ partial | Alacritty supports double underline in app but not in headless mode |
| vt100 | 0.2.1 | ✗ no | Not in VT100/VT220 — only single underline supported |