Key Signatures and the Circle of Fifths
Why some major scales need sharps or flats, and how the circle of fifths organizes all fifteen of them.
Only the C major scale uses just the white keys. Build a major scale on any other note using the same W-W-H-W-W-W-H pattern, and some notes will need to move up (sharp, #) or down (flat, b) a semitone to keep the pattern intact.
The sharps and flats a key needs are written once at the start of the staff — that's the key signature — instead of marking every note individually.
Sharp keys are added in a fixed order: F#, C#, G#, D#, A#, E#, B#. Flat keys are added in the reverse order: Bb, Eb, Ab, Db, Gb, Cb, Fb. Each new sharp or flat key starts a perfect fifth above the previous one (or a fourth, going the flat direction) — which is why arranging all major keys this way forms a circle: the circle of fifths.
You don't need to memorize all fifteen key signatures at once — start by recognizing that the number of sharps or flats tells you which key you're in, and that G major (1 sharp) and F major (1 flat) are the first keys most people leave C major for.