Transposing instruments — written vs. sounding pitch and per-instrument key signature

A transposing instrument's part is written at a pitch differing from concert (sounding) pitch by the instrument's transposition interval, and its key signature is transposed by the same interval — e.g. a Bb clarinet in a piece sounding in C major is WRITTEN in D major: the WRITTEN pitch sits ABOVE the sounding pitch by a major 2nd (a Bb clarinet sounds a major 2nd LOWER than written). A concert-pitch (score) presentation shows all instruments already transposed to sounding pitch with no individual key-signature offsets. Both are valid depending on stated purpose, but a document must be internally consistent about which one it is.

Gould (Behind Bars, transposition tables and written/sounding distinction); Read (Music Notation, equivalent transposition-interval tables).

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