The seven notes that have their own letters are what’s called diatonal in the Western musical tradition. It turns out that however you divide the octave (12 sem

The seven notes that have their own letters are what’s called diatonal in the Western musical tradition. It turns out that however you divide the octave (12 semitones in the West), you can make a lot of music using only some subset of the pitches (7 in the West) and people gravitate towards those subsets because of their familiarity. The rest, those that in the West are notated with sharps and flats and called chromatic tones – the black keys of a piano – are basically for situations where you need extra expressive power. They literally add "color". (For simplicity’s sake I assume the Cmaj/Amin scale here. In non-Romance countries we typically teach basics of music using relative solfege aka “movable do”, where notes always have the same names (Do, Re, Mi etc) no matter what the absolute pitch of the tonic is)