18.4 · The Staff
Key signatures on the page
G major needs F♯, and F major needs B♭. Writing that accidental on every single F would clutter the page, so notation uses a shortcut: the key signature. One symbol at the front of the staff says "every F is sharp from here on", and the music stays clean.
C major: no sharps, no flats
No sharps, no flats, an empty signature. What you see is exactly what you play.
G major: one sharp at the door
The ♯ sits on the top F line and rules every F on the staff, any octave. Notice the written scale needs no accidental next to its seventh note; the signature already said it.
F major: one flat instead
Same idea, other direction. The ♭ sits on the middle B line: every B is flat now.
Quiz
1 / 3A key signature with one sharp on the F line means…