Baritone and sub-bass clefs are historical curiosities, not live choices for Gradus's repertoire

Plain bass clef (F clef on the second-from-top line) is correct for essentially all of Gradus's low-voice and low-instrument writing. The baritone clef (F clef on the middle line, or historically a C clef on the top line) and sub-bass clef (F clef on the top line) are 18th-century-and-earlier notations essentially never used in modern engraving, choral writing, or the instrumental repertoire Gradus teaches (bass, cello, trombone, tuba, bass voice). They should never be a candidate clef in any Gradus rule or renderer.

N/A as an authoring bug (no code path in Gradus produces or accepts these clefs) — included for completeness because the brief specifically asked this chapter to state when baritone/sub-bass clef is 'rarely correct,' and the honest answer is: essentially never, for anything Gradus teaches.

Gould, Behind Bars, Clefs chapter, lists baritone and sub-bass clefs explicitly as obsolete/historical, not in modern use. Read, Music Notation, agrees they survive only in scholarly editions of early music. Ross does not discuss them at all (consistent with their complete absence from 20th-century engraving practice, which is Ross's whole subject). Stone, Music Notation in the 20th Century, likewise gives them no space. All four authorities that discuss the question agree.

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