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Here’s an example from Bach’s Fugue in Ab major from WTC book 1:

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In the last bar of this image you can see they put an Ab in the left hand and a G natural in both the left and right hands. Since this piece is already in Ab and the measure has no accidentals preceding these notes, what is the reason for using the accidentals here? It seems entirely unnecessary. Also, the measure in front of it uses a Gb in the right hand and an A natural in the left hand, so I'm wondering if this is just a courtesy to the reader?

To be clear, this is one simple example, but I've seen this a whole bunch in music from many different time periods. I've seen places where they do this the measure after like in this instance, but obviously there are times when they don't do this too. What are the rules here?

Landon Poch
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    You answered yourself, they are "courtesy accidentals", not strictly necessary, but useful to us who are humans and not computers. – MMazzon Aug 10 '20 at 08:44
  • I suspect these are to make the modulation clear. I think there is a temporary modulation to some key with a A natural for half a bar and the a return to the home key, which has a Ab. – Neil Meyer Aug 10 '20 at 09:03
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    This question was closed as a duplicate, but the accepted answer in the original question does not really explain this well. In both cases, the second accidental is in a different octave than the first. The correct behavior for accidentals in different octaves is not well standardized. See the Wikipedia at https://en.wikipedia.org/wiki/Accidental_(music) : “If a note has an accidental and the note is repeated in a different octave within the same measure the accidental is usually repeated, although this convention is far from universal.” So the “courtesy accidental” is more than just courtesy. – Howlium Aug 10 '20 at 22:25

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