Noteheads a second apart on one stem must be displaced to opposite sides, not overlapped

When a chord contains an interval of a second, the two noteheads cannot sit at the same horizontal position without colliding. One stays on the stem's normal (primary) side; the other is displaced to the opposite side. The notehead closer to the stem's own side stays in the primary column; its second-neighbor is bumped to the far side.

Two noteheads a diatonic step apart rendered at the same x-position (full overlap, illegible), or displaced in the wrong direction.

Gould, Behind Bars (seconds in chords); Read, Music Notation, chord-spelling chapter; Ross, same displacement convention.

This rule needs the notated content and the rendered result together — the score says what should happen, the page says whether it did.