MathRule

A filled horizontal rule centered on the math axis. Its default thickness is 0.04 em.

Property	Default	Meaning
width	em(1)	Rule length, using ordinary Gum sizing.
thickness	em(0.04)	Stroke thickness represented by a filled rectangle.
shift	Absent	When supplied, place the bottom this far above the baseline, instead of centering on the axis.
fill	Inherited color	Optional rule paint override.

The parsed form is \rule[shift]{width}{height}. Its dimensions preserve TeX units: point sizes remain fixed in scripts, while em/ex use the local text size. Nonpositive TeX dimensions have no ink; negative width retains signed advance. The parsed rule is an ordinary atom. A direct MathRule defaults to no atom class and accepts klass, left, and right overrides.

Common font, color, and sizing properties follow Gum units. Math preserves separate logical advance, outline ink, baseline, and math-axis guides. See math authoring for supported TeX and font setup.