Figures stack with the largest interval number on top, smallest on bottom

When more than one figure applies to a bass note, the numerals are stacked vertically in descending order top-to-bottom (a 6/4 chord shows 6 above 4; a seventh chord in third inversion shows 4 above 2; a full 7/5/3 shows 7, then 5, then 3). This mirrors the interval order from the bass upward.

A 6/4 figure is authored or rendered with 4 on top and 6 on the bottom, inverting the standard order.

Gould explicit stacking diagrams; Read (figures one below the other, the higher number uppermost).

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.