tl;dr: The notation system predates the modern, commonly-used 12-Tone Equal Temperament, for which there are (at least two) ways to describe any note within the

tl;dr: The notation system predates the modern, commonly-used 12-Tone Equal Temperament, for which there are (at least two) ways to describe any note within the (12-Tone) octave, either by sharps (D sharp) or by flats (E flat). In 12-TET, there are exactly twelve notes in the octave, and sharps and flats can be said to "overlap". In earlier temperament systems, these notes may have been distinct (or in some cases unavailable), as the relationships between notes were based on non-equal, if not more mathematically perfect, ratios.