2.26One subtle but important idea: accidentals have spelling

At the piano:

F♯ and G♭ are the same physical key.

But theoretically they are not always the same note.

For example, D major is:

D E F♯ G A B C♯

not:

D E G♭ G A B D♭

Why?

A normal seven-note scale uses each letter name once:

D E F G A B C

then adjusts them with sharps or flats.

This makes intervals and chord spelling understandable.

For now you don't need to obsess about spelling, but don't think that F♯ and G♭ are always interchangeable on paper.