An accidental symbol placed to the left_ of a note (see example) indicating that the note is to be raised by two half steps (two semitones).
The double sharp 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 sharp that also has a tie across a barline carries the double sharp 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 sharp will always be shown with two sharp symbols, regardless of the key signature. For example, if the current key signature shows one sharp (i.e. a F-sharp in the key of G Major), adding one sharp symbol to a F-sharp on the top line of the staff would not create a double sharp. Only the addition of the second sharp symbol to that note would indicate a double sharp.
There is a rarely used symbol that combines the natural symbol and sharp symbol. There is no specific name for this, but the natural symbol would be found in a measure with a double sharp to lower the pitch of the indicated note by a half step (one semitone). The natural symbol would cancel the first sharp symbol of the double sharp shown on the previous note in the measure and and the sharp symbol indicate that the pitch would only be raised 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.