In the entry for 28 October 2018, after explaining that the blog concerns only the major dominant ninth chord--such as C: V9, G9, or A9, etc.--not what I call the minor dominant ninth, or dominant 7th plus b9, or any of the non-dominant ninth chords, I wrote:
Still another exclusion from this blog is the dominant ninth with altered fifth. These chords also begin to appear with some frequency in the 1890s. The version with raised fifth is more common; so, in C major G-B-D-F-A becomes G-B-D#-F-A, which happens to form a whole-tone scale pentachord also: D#-F-G-A-B [see the second example under (d) below]. Less common is the dominant ninth with lowered fifth, so: G-B-Db-F-A [see the first example under (d) below]. This one, too, can be spelled in scalar form as a whole-tone pentachord: F-G-A-B-Db.
I'm going to take that back a bit, because the derivations of these chords are of historical interest for the familiar story of their connecting the dominant seventh and ninth with the whole-tone scale. Both appear in music first as alterations of V or of V7.
The dominant seventh with lowered fifth was normally used in second inversion from the 1870s on. The first example under (e) below shows the dominant ninth and its alteration with the lowered fifth first and then the progression with V(b5)7 from which the latter is derived. Two points about it: exploited by Richard Strauss and others influenced by him, V(b5) is sometimes called the "Strauss chord" and normally appeared with the fifth in the bass; although the progression is functionally V(b9)–I, its notes are identical to IV: Fr+6-V.
The chord with raised fifth, as V(#5)--see "from" in the second example under (e) below--already appears in galant-style music in the second half of the 18th century in the form V7(#5)/IV–IV; it has been associated particularly with Mozart, though he was by no means the only person to use it. The version with the ninth is unlikely before 1890.
Here are two simple (made-up) examples of scalar elaboration, showing the whole-tone scale as a melodic figure. At (f), G9 is first, but the scale is over V(b9) with the lowered fifth in the bass. At (g), the fifth is missing in the G9; the raised fifth (D#) appears as leading into E6.
