A mid-system clef change is placed immediately BEFORE the barline it precedes, not after

When a clef changes partway through a system (not at a system break), the new clef sign is engraved at the END of the measure it will govern — immediately before the barline — rather than at the start of the following measure after the barline. The barline itself, and any following key-signature or time-signature change, comes after the new clef sign. This positions the new clef where the reader's eye naturally looks just before crossing into new material.

A mid-system clef change rendered (or hand-notated) immediately after the barline rather than before it — inconsistent with both the recommended convention and with what Gradus's actual rendering stack (VexFlow, Verovio) does natively, so this would actually require SPECIAL, extra effort to get wrong.

Gould, Behind Bars, Clefs chapter, states the before-the-barline placement as standard modern practice. Ross, The Art of Music Engraving, agrees and treats it as unambiguous house style across professional engraving. Read, Music Notation, is the source of the actual disagreement the brief asks this chapter to surface: Read notes that some (particularly older or hand-copied) traditions place the new clef AFTER the barline, at the start of the new measure, treating the clef as belonging to the measure it introduces rather than the one it closes out. This chapter recommends Gould/Ross's before-the-barline convention for Gradus, both because it is the dominant modern convention and because it is what VexFlow and MEI/Verovio both implement by default (a mid-system clef change renders attached to the end of the preceding measure in both rendering engines) — fighting the renderer's native placement to implement the minority convention would cost real engineering effort for a stylistically inferior result.

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