Original sentence: In the 18th-century 9th and 11th chords where theorized as downward extensions of 7th chords, according to theories of supposition.In the 20th century, especially in jazz and popular music, 9th chords where used as elaborations of simpler chords, particularly as substitutes for the tonic triad at the end of a piece
Edited sentence: In the 18th-century 9th and 11th chords were theorized as downward extensions of 7th chords, according to theories of supposition.In the 20th century, especially in jazz and popular music, 9th chords were used as elaborations of simpler chords, particularly as substitutes for the tonic triad at the end of a piece