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Can a chord contain both the C and C♯ notes? Or do you always have to use D♭ rather than C♯ in a chord that already contains the note C?

I'm guessing the answer is no, both C and C♯ should not be allowed - but I do see a lot of inconsistency on the web with what enharmonic is used to describe a chord.

For instance, take the A♭m chord. I'm pretty sure it is correct that this chord contains the notes A♭, C♭ and E♭ ? In many places I look online, they say the C♭ is a B. But I don't think that is right, because in the A♭ major scale, the B is flat.

So therefor, going back to my question, a chord in the key of A♭ can never contain a "straight" B note. The B must always be one of B♭, B𝄫 or B♮. And if you're talking about a note that represents the 3rd, then that note must always start with 'C' - and therefor the minor third in Ab major can never be described as B.

Correct?

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The diatonic third in A♭ is always going to be some kind of C, because B♭ is already the diatonic (major) second and, as mentioned, when naming chords in a diatonic scale each note must be assigned a different name. A♭m will use the minor third, C♭ (which is enharmonic with B), while A♭M will use the major third, C. –  Rein Henrichs Aug 29 '11 at 17:26

6 Answers 6

I can think of at least one example where I encountered a C and C# conjoined. In Béla Bartók's Nine Little Piano Pieces, you'll find them in the Menuetto from measure 10 on.

It would be natural to find more of these in Bartók, especially the pieces using polytonality. Other composers using polytonality like Stravinsky should also have them. So, look for composition of the early 20th century using polytonality.

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In classical Western music theory, each diatonic scale contains seven notes, and each of the notes must be assigned a different note name. (So one also does not write CX for the D in the C major scale.)

In non-equal temperament, C# and Db may in fact be two different pitches, and a diatonic scale that contains a C must not also contain a C#.

In more modern music theory, uses of polytonality and non-diatonic scales in composition, especially the chromatic scale with 12 tones and the diminished and bebop scales with 8 tones, make it impossible for a scale to contain only notes with distinct note names. That and the widespread use of equal temperament (so that C# and Db are indeed enharmonic equivalents) makes it slightly more acceptable to refer to the third note in the ascending Ab minor scale as a B; though perhaps this practice is for the sake of players (especially piano players) less trained in music theory that thinks of sharps and flats as only modifiers to change from white keys to black keys.

I do think the vast majority of musicians performing Western music will prefer you spell Abm as Ab Cb Eb, and not Ab B Eb.

But going back to your original question: I don't see why not (theoretically). The presence of a minor second or a minor 9th is a big aesthetic no-no in a lot of chord-voicing traditions, but that rule may not be so hard and fast. In theory, the same notes may even be interpreted differently functionally by fitting them over different scales. For example, the notes C-E-G-Bb-D# spell the usual C7+9 chord. But reading the D# as an Eb, those are the root, 4th, 6th, 8th, and 3rd notes of the C diminished scale.

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In jazz, a sharp 9 chord is often spelled with both thirds emphasizing the bluesey major/minor sound thus A7+9 = A C# E G C though the last C would technically be a B# according to the rules of western harmony.

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A 7#9 chord doesn't have both thirds. It has a major third and a sharp 9. The sharp 9 is enharmonic with the minor third but functionally quite different. –  Rein Henrichs Aug 30 '11 at 20:03
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@ReinHenrichs: correct. There is something to this answer though: you can voice A7+9 with both the b9 and the #9. There you have A, Bb, B#/C, C#. I would prefer to spell it A - C# - G - Bb - C, thus having both C and C#. This is really a sign of the limitations of our notation system, in my opinion. –  Gauthier Apr 22 '13 at 18:09
    
@Gauthier: The issue is that the 7#9 chord is often considered to be constructed from the 7th mode of the (ascending) melodic minor scale, but Bb melodic minor has a Db rather than a C#. –  Rein Henrichs Apr 22 '13 at 18:49
    
Agreed. And having a Db instead of a C# in a A7 variant of chord feels... urk. This is what I meant with limitations of the system. You can see A7+9 as a chord with the third, the seventh, a minor ninth, and an augmented ninth, then come up with one spelling. Or you can see it as a set of tones from Bb minor melodic and come to another spelling. There are several ways to look at it, and they are not very consistent. It is even worse when you start using things such as diminished scales (on A7b9. Too many tones!) or whole tone scales (too few!). –  Gauthier Apr 23 '13 at 13:49

For instance, take the A♭m chord. I'm pretty sure it is correct that this chord contains the notes A♭, C♭ and E♭ ? In many places I look online, they say the C♭ is a B. But I don't think that is right, because in the A♭ major scale, the B is flat.

A minor triad is a minor third then a major third. A♭ to C♭ is a minor third and C♭ to E♭ is a major third. A♭ to B is an augmented second and B to E♭ is a diminished fourth. They'll both sound the same when you play them on a piano or guitar, but if you write them down the second one would be considered wrong.

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Well, sure. As a chord is simply any number of notes defined in terms of their interval from a "root" note, a chord can have any two or more semitones in it. A lot of contemporary composers such as Eric Whitacre play around a lot with dissonant intervals and "cluster chords" involving minor-second intervals like C/C#.

More realistically, it would be rare to see both a C natural and a C# in the same chord, notated as such. This is primarily because the notes would take up the same line or space in the staff, so it would be difficult to sightread and could be impossible to notate properly if there's a D or B in the chord as well (as in the aforementioned cluster chords). However, C# and Db are enharmonic (same note on the piano keyboard), so you may see a C and Db in the same chord. In the key of Ab, an Ab major triad with the fourth added (not a suspended chord, but an add4) would have both of these tones, and in the contemporary climate of music composition it wouldn't be that odd to hear.

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Check out Chopin's etude op 10 no. 2 in A minor. He uses this kind of device all throughout. In fact, it's just what you're taking about, a C and C# at the same time.

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