Above-staff and below-staff ledger lines follow identical geometry

There is no engraving distinction in length, thickness, or spacing between ledger lines drawn above versus below the staff — the same continuation rule applies symmetrically in both directions. The only real differences are contextual: stems on below-staff notes point up (per the middle-line stem-direction pivot, see the Stems and Flags chapter) so they reach toward the staff, and below-staff ledgers are far more likely to collide with a lower stave in a grand staff or open-score system, while above-staff ledgers are more likely to collide with a slur, tie, or dynamic hairpin arcing above.

A renderer or hand-drawn engraving that uses a shorter or thinner ledger line below the staff than above (or vice versa) for no musical reason — a pure implementation inconsistency, since the SMuFL/VexFlow/Verovio glyph is direction-agnostic by construction.

Gould, Behind Bars and Ross, The Art of Music Engraving both describe leger lines as one symmetric rule, not two.

This rule is about where marks land on the page, so it can only be judged after engraving. Collision and spacing faults live here.