Double flat

It lowers the written note by a whole step. A flat ♭ lowers a note by one semitone, the distance from one piano key to the very next one, black or white; the double flat lowers it by two. It sits just before the note on the staff, like any other accidental (a sign that raises or lowers a note), and holds for every note on that line or space until the end of the bar.

Some everyday examples: E𝄫 sounds like D, B𝄫 like A, and C𝄫 like B♭. To cancel it, musicians write a natural ♮, or a single flat if they want the note only one semitone below its natural pitch.

Why not just write the simpler note?

Because each letter has a job in the chord or scale. A C diminished seventh chord (a tense four-note chord built by stacking notes a third apart) is spelled C–E♭–G♭–B𝄫: stacked thirds need the letters C, E, G and B. Writing A instead of B𝄫 gives the same sound on a piano but hides how the chord is built. Double flats often turn up in keys that already have many flats.

The flat itself began in medieval chant as a rounded letter b, called b rotundum or b molle ("soft b"), marking a lowered B. The double flat is simply that sign doubled, which is why it looks like two flats squeezed together.