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I have recently learned the principles of the standard twelve-tone octave applied to major and minor scales. I understand each scale is defined by its main note, and the pattern of intervals, of half versus full tones, ascending the octave.

The C-major scale, for example, shares with other major scales the interval relationship that the half-tone interval mediates the third and fourth tones, as well as the seventh and eighth (final) tones, with all other intervals being full tones.

Since musical structure is carried essentially by the relationship among tones (the ratio of physical frequencies between any two tones), not by a tone itself (the absolute physical frequency of any single tone), it would seem to follow that any composition based on the tones of the C-major scale may be readily translated to any other major scale with full preservation of the musical structure. Of course, many compositions are described as following a major key other than C. In all, twelve major keys exist, corresponding to the twelve tones into which the octave divides.

In musical terms, artistic or technical, what is the reason for all compositions using a major key not simply using the same particular scale? For example, what is the reason for some compositions being written in C major, while yet others in G major, if both keys share the same essential musical structure?

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  • Essentially the question is the same. A further question seemingly not addressed by the responses is why the social conventions have not emerged such that they key would be chosen through the arrangement, or by a soloist, or the members of quartet, and so on, instead of the composer prescribing to the performers that one of the keys is correct.
    – brainchild
    Commented Feb 20, 2022 at 2:52
  • The second paragraph describes the intervals of the minor scale while claiming to describe the C major scale. As to the "further question," composers typically do specify a key. Whether performers are at liberty to transpose them to other keys depends on several factors. I doubt anyone has ever performed Mozart's Requiem in G minor, for example.
    – phoog
    Commented Feb 20, 2022 at 6:41
  • Try singing Happy Birthday in C major. Then sing it in F major. Compare and analyze these as subjective experiences. Did it feel exactly the same in both keys, or was something different? Commented Feb 20, 2022 at 14:44
  • It's interesting that the duplicate contains nothing taking a historical overview of "how we got here." Because that's a big part of the answer: We're working with trends set in motion in medieval days. I'll try to contribute an answer there soon, but the short version is: At first (plainchant, early renaissance) nobody cared much about instrumentation or absolute pitch. It really was infinitely transposable. Ca 1600s-1800s, it started to matter a great deal what instrument a piece was for (thus the physicality), plus pitch became standardized, plus all the temperaments mattered. Commented Feb 20, 2022 at 20:23
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    Today there definitely are genres and cultures where key is no longer fixed, where the performers pick it at will and transpose as necessary. Jazz—what's the official key of Mack the Knife?—pop, etc (do you tune your "two turntables and a microphone" to A=440?). Commented Feb 20, 2022 at 20:25

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A composition doesn't "breathe" until it's performed either on instruments or by vocalists. Changes to the key signature create opportunities to match the optimal pitches for either. A vocalist or instrumentalist may perform horribly in one key, but fantastically in another.

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Since musical structure is carried essentially by the relationship among tones (the ratio of physical frequencies between any two tones), not by a tone itself (the absolute physical frequency of any single tone) (...)

This is not true. Even people without perfect pitch perceive transposed music differently, simply because it's played higher or lower.

In musical terms, artistic or technical, what is the reason for all compositions using a major key not simply using the same particular scale?

Because arranging music is often a substantial creative work (even if arrangement itself not copyrighted). The composer chooses pitch of the notes for the effect they want, as well as takes advantage of the characteristics of the instrument. Changing the key involves redoing this work. It might be difficult or even impossible for given instrument, or may require creative choices of changing notes (e.g. changing voicings, chord inversions, even reharmonizing). The performer may not have time or necessary skills to do it.

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    Listeners are generally able to perceive differences in pitch, yes, even without a mutual reference, if the difference is adequate. The premise I gave is that such differences do not account for a difference in musical structure, which is given only by the differences among the tones within the same performance. Thus all major scales carry the same essential musicality, not shared with minor or other scales.
    – brainchild
    Commented Feb 20, 2022 at 4:24
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    This is addressed by the answers: music.stackexchange.com/a/15225/63781 music.stackexchange.com/a/15250/63781 and others. Consider this remix: youtube.com/watch?v=jEOPp4iV1WQ Is its "essential musicality" the same as of the original? If you played it on a disco would you expect the audience to react the same? Commented Feb 20, 2022 at 5:09
  • Is a composer's choice to use C major versus D major substantially determined by an understanding of the practical physical properties of the target instrument, more so than of the musical and emotional concepts embodied in the work? It seems doubtful to me, as writing music for a particular instrument or arrangement is by no means universal among composers' techniques. Don't many write music as a pure form, with the instrumental properties considered only later?
    – brainchild
    Commented Feb 20, 2022 at 5:41
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    @epl This must vary a lot from case to case. I'm fairly sure Chopin's Piano Concertos were composed for the piano. Good composers learn about the instruments they compose for, to make sure what they write is playable and sounds the way they wish. Songwriters need to have an idea of vocal ranges, especially if they write for a specific singer. etc. A counterexample might be transposing jazz standards, which is a common practice, but these tunes were often composed as pop or music theater compositions with a specific key in mind, and it's jazz musicians who are skilled in rearranging them. Commented Feb 20, 2022 at 6:00
  • True, there's a big, discernible difference between a song in, say, C as opposed to G, sung or played on any instrument, but that same song in C or C#? I'd argue that well over half the people, including musos, would be able to tell, unless they had the both back to back.
    – Tim
    Commented Feb 21, 2022 at 10:12

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