Stem direction pivot must equal the true middle line of the clef

A note's stem points down if the notehead is on or above the staff's middle line, and up if it is below the middle line. Stated identically by Gould (Behind Bars, Stems chapter, pp.16-19) and Read (Music Notation, Stems chapter). The middle line differs by clef: treble = B4, bass = D3, alto = C4 (middle C sits ON the alto middle line — the entire reason alto clef exists), tenor = A3.

Gould, Behind Bars; Read, Music Notation, Stems chapter; Ross, The Art of Music Engraving, stem-direction section — all three agree on the pivot-at-middle-line rule and the on-the-line-is-down tie case.

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