Accidentals are symbols that indicate the alteration of the pitch of a tone.
A double sharp raises the pitch of a tone by two half steps.
Notice in the musical examples above, the accidentals are always placed in front (to the left) of the notes they affect. On the other hand, when writing about music, the word or symbol representing the accidental follows the letter name (e.g., “A flat” or “G#”).
The black keys of the keyboard can be labeled with either flats or sharps.