Correct music notation generally involves the rule of not conflicting with the key signature of the music.

In the following examples, the music is composed in C#min which of course has 4 sharps C#, D# F# and G#

The composer deliberately avoids scoring a C natural as this would contradict with the C# determined by the key signature, so the composer writes B# instead. The same with a G natural, the composer instead writes is as a double-sharp F, viz: ' xF '.

Some examples of this are shown below:

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