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I wrote a sone in Dd Major, but I could also notated that it would be equivalent to say C# Major as well. I am not well versed in musical theory and I think both are equivalent to each other and wonder if there are situations which there are preferences to say one over the other. If not why. I play guitar and bass so I would love to know.

What I am mainly writing for is guitar, bass, vocals, keys and drums. But there will be some translation from Keys to actual stringed instruments like Violins and Violas with possibly some oboe or clarinets. I have an EWI player too interested in subbing for those wind parts so I would want to make sure he is comfortable with the scale notations.

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If you wrote it out as Db, but put 2 # for the key signature, it'd work in D. If you wrote it out in C#, but put no # or b at the beginning, it'll be in C. Is there a particular reason it has to be in C#/Db ? After your edit, I feel that either C or D has to be a more playable key.I appreciate that this doesn't directly answer your question, so it's a comment. – Tim Feb 10 '14 at 15:17
I am using the C#/Db scale because it is easier for the vocalist to sing the melody in. Plus it was exciting for me not to write in C or D as a guitar player as it forced me to think of different positions for chordal progressions. FYI. – tony.stack Feb 10 '14 at 17:12
For what it's worth, I know players, particularly horn players, who will sit out if they're handed a chart in C#. Yes, it's enharmonically equivalent, but in most contexts, Db is more commonly used, and a lot of folks aren't comfortable reading in C#. – kiprainey Feb 12 '14 at 4:58

C# and Db are enharmonically the same. Meaning, they are played by the same key on a piano, but they have a different musical meaning and they actually should sound a tiny bit different (altough the difference is minimal). Yet, string instruments or woodwinds one might be able to play them slightly different and thus correct.

In pythagorean tuning, each semitone consists of 100 cents on average. C# and Db actually differ 41 cents from eachother. This stems from the fact that the tuning is defined by the going through the circle of fifth with a ration of 3:2 (see table here).

The interested reader can refer to the fantastic book "Music: a Mathematical Offering" by Dave Benson, available for free here. "

Though to answer your question in practice: usually the key with least signs is used : )

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Good answer. It's tempting to think of everything in equal-temperament terms, but many instruments are capable of playing in just intonation. – Jonathan Van Matre Feb 11 '14 at 19:55
Thanks Jonathan. Many people forget about the existance of cents and how tones are not equal : ) Music is fluid : ) – dorien Feb 12 '14 at 11:37

You're right that in most situations those are absolutely equivalent, so the distinction has more to do with what instruments will be playing it and what they're used to. If you're writing for piano, it really doesn't matter much at all, both are pretty common. All things being equal, I might slightly prefer to call it Db just because it has fewer accidentals in the signature. If string instruments will be playing it, I would first note that isn't a very resonant key for them and might be better if you transposed it. But I would lean toward C# maybe since—very generally—string players prefer sharps to flats. I would say the opposite is true for most wind instruments.

If you clarify what you're writing for, it would help, but the general answer is that it doesn't make a huge difference, and the default choice would probably be to use Db with it's five flats rather than C# with it's seven sharps (including notes like E# and B#, which tend to throw beginners off more than others).

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They are indeed equivalent, at least in Equal Temperament (which is the most widely used tuning system in western music). You might prefer one over the other depending on how things modulate.

If you're going to modulate to the parallel minor, use C#, since C# minor has 4 sharps, whereas Db minor doesn't really exist (it would have 8 flats--one for each note and then a second one for B, putting Bbb in the key signature).

If you're going to modulate to the dominant, use Db, since the dominant is Ab with 4 flats, whereas the dominant of C# would be G# with 8 sharps.

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In just intonation there is a real difference between the two, in equal temperament not. For fixed pitch instruments there is no difference, but for other pitched instruments (including vocals) it should make a difference. That does not imply that a performer on a non-fixed pitch instrument will indeed make that difference heard, all the more so when that performer is playing together with fixed pitch instruments.

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