Forum: music.stackexchange.com
Tags: <theory><chords><chord-theory><harmony>
Title: Why does Cdim7 contain an A instead of a Bb?
Question: CM7 - C, E, G, B  C7 - C, E, G, Bb  Cm7 - C, Eb, G, Bb  Caug7 - C, E, G#, Bb  But...  Cdim7 - C, Eb, Gb, A.  Why Does Cdim7 contain an A and not a Bb?

Answers:
sents: text: C°7 actually includes a B doubleflat .
text: A major seventh above C is B and a minor seventh above C
text: is B♭.
text: This means that a diminished seventh above C is actually B♭♭, which is enharmonic to A.
text: But the seventh is not A, because A is just a sixth above C. A is enharmonic to B♭♭, but since we want the seventh above C, it must be B♭♭, not A.
text: But it gets more interesting:
text: since fully diminished sevenths are just stacked minor thirds, they can be spelled four different ways, with each pitch as root; we say that these chords are enharmonic, just like we say that A is enharmonic to B♭♭.
text: This then means that C–E♭–G♭–A is a diminished seventh chord, but with A as the root: A–C–E♭
text: –G
text: ♭.
text: If E♭ is the root, we'd rather spell it as D♯–F♯–A–C.
text: If G♭ is the root, we'd spell it as F♯–A–C–E♭.
text: These chords all sound the same as C°7, but they are spelled differently.
answer_details: author: music.stackexchange.com/users/21766/Richard
score: 13
sents: text: Other answers have pointed out that it's a B♭♭, not an A.
text: To answer the question "
text: why not a B♭":
text: The chord in question is the chord of the diminished 7th.
text: As David Bowling and Richard have stated, if the root is C, the diminished 7th is B♭♭.
text: The chord is also called the "diminished 7th chord".
text: This name and its chord symbol Cdim7 are perhaps confusing.
text: I don't know if this is what you thought, but, just for the record, "Cdim7" does not
text: mean
text: the diminished triad C-E♭-G♭
text: as indicated by "Cdim" the pitch B♭ as indicated by "7", just as C7 means a C triad plus B♭
text: It's not a [ diminished triad ] with a seventh.
text: It's a chord of the [ diminished seventh ].
answer_details: author: music.stackexchange.com/users/29126/Rosie F
score: 6
sents: text: A Cdim7 chord is actually spelled C-Eb-Gb-Bbb, where the interval from C to Bbb is called a diminished seventh.
text: The interval from C to A is a major sixth, but sometimes you do see people spell Cdim7 as C-Eb-Gb-A for convenience to avoid the double flat, it is just technically incorrect.
text: This chord is sometimes called a fully-diminished chord, in contrast to a half-diminished chord.
text: A half-diminished C chord ( Cmin7b5 , C-7b5 , or C ∅ ) is spelled: C-Eb-Gb-Bb.
text: These are often found in ii-V-i progressions in jazz.
answer_details: author: music.stackexchange.com/users/39614/ex nihilo
score: 5
sents: text: It actually contains a Bbb (double flat).
text: Often (mis)spelt as A for convenience.
text: You'll also often see it notated as C, Eb, F#, A.
text: A dim7 chord is a pile of minor 3rds.
text: A minor 3rd up from Gb is Bbb.
answer_details: author: music.stackexchange.com/users/17003/Laurence Payne
score: 1
sents: text: Adding a little to the others.
text: Cdim7 cannot contain a Bb note.
text: Bb only produces a minor 7th from the root.
text: To make it a diminished 7th, the space between root and m7 needs to be one semitone smaller.
text: That's the reason it gets called Bbb.
text: It sounds just like A, but since C>A is a M6, that wouldn't be correct if the chord was to be called dim7.
answer_details: author: music.stackexchange.com/users/2808/Tim
score: 1

Summary of the answers:
Since fully diminished sevenths are just stacked minor thirds, they can be spelled four different ways, with each pitch as root; we say that these chords are enharmonic, just like we say that A is enharmonic to B♭♭. A  Cdim7  chord is actually spelled C-Eb-Gb-Bbb, where the interval from C to Bbb is called a diminished seventh. The interval from C to A is a major sixth, but sometimes you do see people spell  Cdim7  as C-Eb-Gb-A for convenience to avoid the double flat, it is just technically incorrect.
Sometimes people Cdim7  as C-Eb-Gb-A for convenience to avoid the double flat, it is just technically incorrect.