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This question already has an answer here:

Is it just a convention? Or there is more to it ?

marked as duplicate by guidot, MattPutnam, neilfein, Richard, Matthew Read Mar 20 '17 at 18:50

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    Why would you call a key with a gb, G major? Why not Gb major? – Neil Meyer Mar 20 '17 at 17:25
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Because then you would have 2 Gs in the scale. In any 7 note scale, we want 1 of each letter name so having the scale be G A B C D E F♯ G makes sense. See this question for more information about how this works.

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Because the whole point of a key signature is not having to use any additional accidentals as long as you keep within the tonality specified by your key signature.

Writing G major as g-a-b-c-d-e-g♭ would mean that you constantly have to switch between g♮ and g♭ sharing the same location in the staff while the location of f goes unused. That is not helpful for singing scales. You want the "home scale" to work out one note space after the other without interspersed accidentals.

For G major, this means using f♯ rather than g♭.

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The major scale is just a modern implementation of the Pythagorean diatonic scale built up from the following intervals:

  • G: some arbitrary base pitch
  • A: pure fourth down from D
  • B: pure fourth down from E
  • c: pure fourth up from G
  • d: pure fifth up from G
  • e: pure fifth up from A
  • f♯: pure fifth up from B
  • g: octave up from G

The reason we use all fourths and fifths: those intervals are distinguished by a clear, simple physical relationship: e.g. D has ³⁄₂ the frequency of G, and C has ⁴⁄₃ the frequency of G etc.. At these simple integer ratios, the notes' harmonics match up, which gives a very clear consonant result.

The reason we stop at seven notes is that further notes, such as pure fourth down from f♯, would be surrounded on both sides very narrowly (in modern terminology, by a semitone) by other notes, and that is a bit weird melodically.

So, that is the scale from which the letters are taken. The names fourth and fifth reflect the counting of four/five respectively notes from interval-start to interval-end. Thus is would make no sense to label the f♯ degree, being the fifth note up from B, as something with G in its name (which would be a sixth up).

  • The other answers cover it. But perhaps another angle might be helpful: yes, it's just a convention (in modern equal tempered tuning) that G major has an F# instead of a Gb. But it's a convention that's useful, because it makes reading and writing a standard major scale clearer. In this convention, every scale step in a major scale is a different letter from A to G, in the form of a flat, a natural, or a sharp. But yes, it's "just" a convention. – Scott Wallace Mar 21 '17 at 12:46

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