The double flat, in bass clef

The double flat, 𝄫, lowers the natural note by two semitones: B comes out sounding like A, and D like C. It appears for the same reason as the double sharp: so that the letter fits the scale even when the key does not.

This lesson is written in bass clef on purpose. Accidentals work the same on both staves — the sign always goes in front of the note, at the same height — but it is worth checking that you read them as quickly down here as up there.

That closes the accidentals: sharp, flat, natural and their doubles, on both staves.