An accidental sign consisting of two flat symbols that lower a note by two half steps (two semitones).
The double flat symbol alters the pitch of the note to which it is attached as well as any subsequent occurence of the same note (identical line or space) in the same measure. Notes with the same pitch name, but a higher or lower octave, are not effected. Any note with a double flat that also has a tie across a barline carries the double flat to the note on the other side of the barline. Notes in the new measure that are not tied to altered notes from the previous measure revert to their original pitch and are performed using the current key signature. It should also be noted that a double flat will always be shown with two flat symbols, regardless of the key signature. For example, if the current key signature shows one flat (i.e. a B-flat in the key of F Major), adding one flat symbol to a B-flat on the third line of the staff would not create a double flat. Only the addition of the second flat symbol to that note would indicate a double flat.
There is a rarely used symbol that combines the natural symbol and flat symbol. There is no specific name for this, but the natural symbol would be found in a measure with a double flat to raise the pitch of the indicated note by a half step (one semitone). The natural symbol cancels the first flat symbol of the double flat shown on the previous note in the measure and and the flat symbol indicates the pitch would only be lowered by a half step (one semitone). In the examples below, the notation on the left is the preferred notation since it indicates the same pitches and is easier for the performer to read.