Borrowing the dominant's pull for a chord other than tonic. Learn tonicization — making a non-tonic chord sound briefly like "home" — through secondary dominant chords (V/V, V7/ii…) and secondary leading-tone chords, then part-write both correctly in four voices.
Six ways to master Unit 7 — pick whichever fits how you like to study.
Four topics from the College Board CED, in order.
Unit 7 introduces tonicization — briefly treating a chord other than the tonic as if it were "home." The tool for this is a secondary (applied) dominant: borrowing the V (or V7) of a target chord and inserting it right before that chord, e.g., V/V ("five of five") momentarily tonicizes the dominant.
You'll also meet secondary leading-tone chords — applied diminished triads and sevenths that do the same job with a different sonority — and part-write both types correctly in four voices, paying close attention to how their raised, chromatic tones (tendency tones borrowed from the target key) resolve.
Unlike most AP courses, the College Board does not assign a percentage of the exam to each unit — the exam is weighted by section (45% multiple choice, 45% free response, 10% sight-singing). Secondary function is one of the most heavily tested harmonic-analysis skills on the free-response section.
Unit 7 develops two of the four AP Music Theory Big Ideas: