Editing Extended chord (section)

===18th century===
{{Anchor|18th century}}
In the 18th century, ninth and eleventh chords were theorized as downward extensions of [[seventh chords]], according to theories of '''supposition'''.<ref name="Grove">{{cite encyclopedia|editor-last=Sadie|editor-first=Stanley|date=1980|title=Ninth chord|at=vol. 13, p. 252|encyclopedia=The New Grove Dictionary of Music and Musicians|isbn=1-56159-174-2}}</ref>

In 1722, [[Jean-Philippe Rameau]] first proposed the concept that ninth and eleventh chords are built from seventh chords by (the composer) placing a "supposed" bass one or two thirds below the [[fundamental bass]] or actual root of the chord.<ref name="Grove S">{{cite encyclopedia|editor-last=Sadie|editor-first=Stanley|date=1980|title=Supposition|at=vol. 18, p. 373|encyclopedia=The New Grove Dictionary of Music and Musicians|isbn=1-56159-174-2}}</ref> With the theoretical chord F–A–C–E–G–B the fundamental bass would be considered C, while the supposed bass would be F.<ref name="Grove S"/> Thus the notes F and A are added ''below'' a seventh chord on C, C–E–G–B, triadically (in thirds). This is also referred to as the "H chord".<ref>{{Cite web|title=extended tertian harmony|url=https://music.red-carlos.com/wp-content/pile-jetting-rodqb/extended-tertian-harmony-5895b3|access-date=2021-05-01|website=music.red-carlos.com}}</ref>

The theory of supposition was adopted and modified by [[Pierre-Joseph Roussier]], [[Friedrich Wilhelm Marpurg]], and other theorists. [[A. F. C. Kollmann]], following [[Johann Kirnberger]], adopted a simpler approach and one closer to that prevalent today, in which Rameau's "supposed" bass is considered the fundamental and the ninth and eleventh are regarded as transient notes inessential to the structure of the chord.<ref name="Grove S"/> Thus F–A–C–E–G–B is considered a seventh chord on F, F–A–C–E, with G and B being [[nonchord tone]]s added ''above'' triadically.<ref>{{Cite book|url=https://books.google.com/books?id=_DBLAAAAYAAJ|title=Jazz Player|date=1993|publisher=Dorn Publications, Incorporated|language=en}}</ref>