Page 164 of Pilhofer and Day, Music Theory for Dummies, 3rd Edition, John Wiley & Sons, Inc., 2015 describes a "seventh flat ninth piano chord". Why is the name of an instrument, "piano", part of the name of the chord? I do not see any name of an instrument in the name of any other chord in the book. Does it mean the chord is particularly well suited for the piano? The book also later refers to it as just the "seventh flat ninth chord".
...or take a
C7chord and add a diminished ninth to it...
A diminished ninth is actually enharmonically equal to an octave. So adding a diminished ninth actually just results in the same chord
Obviously they mean add a minor ninth to a dominant seventh chord. You can call that a dominant seventh, flat nine chord and it gets the symbol
The word "piano" in the heading just means to play it on piano. You can play a
7♭9 chord on any chord instrument or ensemble, so this type of chord isn't particular to the piano. (The book may present particular piano voicings of the chords.)
For what it's work there is a diminished ninth chord. It would be a fully diminished seventh chord with an added major ninth. Rooted on
C is would be
C E♭ G♭ B𝄫 D labeled
...when you take the first ...of the major scale...
The "first" of a scale? That's called the tonic. More importantly when talking about chord intervals you general refer to a root, especially when the context is generic like this defining of a single chord type. If you describe chords built on specific scale degrees, I would expect there to be some harmonic context with other chords or some concern about the chord within a key.
Personally, I would get a different book. This level of sloppy terminology in a published book is not acceptable.
I was able to google the book (see the image), but this doesn't help me to understand it. Perhaps it's a mistake, or they refer to the X7b9 chord symbol?
Anyway, it X7b9 is a popular chord that can be played basically on any instrument capable of playing chords. Also the chord symbol is universal, nothing specific to piano about it.
Later in the book (Appendix B) they refer to "piano chords" as to diagrams on keyboard. Interestingly, in the pages I was able to access at books.google I couldn't find an example of X7b9 chord.
The chord consists of major third, perfect fifth (frequently omitted), minor seventh and minor ninth. For example C7b9 is: C-E-G-Bb-Db. If you omit the root of the chord (note C) what is left is a diminished seventh chord, frequently used as a substitute.
It is a poorly written passage. Some texts will take care to suggest specific voicings (different arrangements of notes in any given chord) for keyboard textures because realizing harmony on a keyboard has been such a big part of learning music. The other texture and usually the primary one harmony texts address is choral, but then one person cannot play the harmonies by themselves. Get a better book.
Interesting! you mention a flat ninth and the image copied by user 1079505 shows a diminished ninth: This is really confusing in a book for dummies! ;)
But to me this case is absolutely clear that they mean if you "diminish" a major ninth adding a flat you get a minor 7th.
Now, this is not your problem.
Then why is only this chord named after an instrument, the piano, but not any other chord (in the book) (which presumably can generally be played on a piano too.
You ask why they mention here a piano chord. All chords are piano chords or guitar chords and as an other answer says it can be played on any other string instrument or keyboard. So the word piano is fully obsolet and we are happy that we have not other problems with theory and practice.
I just think a dummie won't struggle about this note. :)