Underline

Draw a rule beneath a math operand. The operand keeps its baseline, and the rule spans its advance. Strings parse as TeX; ordinary Gum elements also work.

Property	Default	Meaning
children	Empty	Underlined operand.
thickness	TeX rule thickness	Nonnegative line thickness.
fill	Inherited color	Optional rule paint.

thickness defaults to the active style's TeX rule thickness. Three rule thicknesses separate body and line; one more thickness follows the line. fill overrides the rule color, while color applies to the whole expression. Zero thickness omits the rule and its padding. Negative thickness is invalid.

The result is an ordinary atom, equivalent to \underline{…}. Use Overline for a rule above the body, or Accent for an under-arrow or tilde.