double bémol

The musical note "B" preceded by two flat (♭♭) symbols. The effect of each flat symbol (♭) lowers the pitch of the indicated note a semitone (or a half step), so B double flat is the pitch "B" lowered by two semitones (two half steps). The resultant pitch would sound the same as the pitch "A".

It's used when the composer needs to lower a node that has already been lowered with a flat in the key signature. For instance if the key signature is a flat major the composer needs to alter E flat.

The double-flat symbol (♭♭) is placed before a note like other accidentals. While single flats usually point to black piano keys, double-flats often point to piano naturals; an Ab is a black key, but Abb is the G natural key (see enharmonic notes).

In music notation, B-double-flat is written by adding a double-flat sign (♭♭) to the note B. On a piano, B-double-flat is located on the white key between the middle and right black key in each group of three black keys.

The purpose of double sharps and flats in key signatures is to represent this scale in the way it is written, and avoid constant use of accidentals on a note - as per the example switching between G and G#.

Yes A double flat is enharmonically equivalent to G, but it is not G. Its an A lowered by 2 half steps. While the frequency will be the same as a G, it is not G. I stress this because its important to distinguish it from G, so that if you need to use it, you will know when to use it.

The purpose of double sharps and flats in key signatures is to represent this scale in the way it is written, and avoid constant use of accidentals on a note - as per the example switching between G and G#.

The key of B♭ major has two flats, B♭ and E♭.

Double accidentals raise or lower the pitch of a note by two semitones, an innovation developed as early as 1615. This applies to the written note, ignoring key signature. An F with a double sharp applied raises it a whole step so it is enharmonically equivalent to a G.

Chords have characteristics. Personalities, almost. The composer Schubert wrote a book about this, in which he characterized B Flat Major as ”Cheerful love, clear conscience, hope aspiration for a better world”.

The purpose of double sharps and flats in key signatures is to represent this scale in the way it is written, and avoid constant use of accidentals on a note - as per the example switching between G and G#.

Notice that with the double flat, you actually have two flat notes on the music instead of just the one "x" indicating a double sharp. Just like a double sharp, a double flat will always come right before the note that the double flat is to be applied to.