Mathematical typesetting axis

The mathematical typesetting axis, or Math axis, is a horizontal alignment reference for a formula. Its offset from the text baseline is a mathematical font constant, represented by axisHeight in an OpenType MATH table.^[ref]

For an invented coordinate example, choose upward-positive coordinates, put the text baseline at $y=0$, and set the axis offset to $a=250$ font units in a computer font with 1000 font units per em. At 20 pixels per em, the axis is $250(20/1000)=5$ pixels above that baseline. A symmetric enclosing shape spanning $y=-350$ to $y=850$ has midpoint $250$, so its centre lies on the axis.

The typeset fraction bar is another useful alignment cue. The reference fixes relative placement; translating the whole formula also translates its baseline and axis.