diff --git a/src/libvterm/doc/seqs.txt b/src/libvterm/doc/seqs.txt
index 164120c..c03fa51 100644
--- a/src/libvterm/doc/seqs.txt
+++ b/src/libvterm/doc/seqs.txt
@@ -167,6 +167,7 @@
 123   SGR 1            = Bold on
       SGR 3            = Italic on
 123   SGR 4            = Underline single
+      SGR 4:x          = Underline style
 123   SGR 5            = Blink on
 123   SGR 7            = Reverse on
       SGR 9            = Strikethrough on