Complex sentence: In it's standard form, it features a three note chord or triad juxtaposed over the top of another, different chord, extending the chord beneath by adding extra pitches to it.
Split into simpler sentences: In it's standard form, it features a three note chord or triad juxtaposed or placed over the top of another, different chord. The juxtaposed triad extends the chord beneath it by adding extra pitches to it.