Secondary dominants and their leading tones are spelled relative to the tonicized key, not the global key

A secondary dominant's chromatically-altered leading tone must be spelled as the leading tone OF THE TONICIZED KEY, not a pitch-class-convenient letter matching the global key's preference.

In C major, V/ii's raised leading tone of D is spelled Db4 instead of C#4 -- Db has no sensible leading-tone relationship to D (a near-unison), unlike C#.

Universal tonal-harmony convention.

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