Accidentals are symbols placed on the left side of a note head. Depending on the type of accidental used, the pitch of a note is raised or lowered one or two steps. 
 


An accidental temporarily raises the pitch of a note one or two chromatic steps. One chromatic step is an ascent or descent to the nearest neighboring note. A chromatic step is better known as a semitone, or a half step. Two chromatic steps are often called a whole tone or whole step. A note with an accidental often becomes one of the black keys on the piano. The accidental temporarily affects a note for the duration of a measure.
 
The sharp sign indicates that the note is raised one chromatic step. 
The flat sign indicates that the note is lowered one chromatic step. 
 
A natural sign cancels the previous sharp or flat. This returns the note to its original pitch. The original pitch of a note is one of seven white keys within an octave. 
 
A double sharp indicates that the note is raised two chromatic steps. 
 
A double flat indicates that the note is lowered two chromatic steps. 
 

HISTORY

Accidentals were first used in the fourteenth century in a practice called musica ficta. The French and Italians were the first to use such a technique by which a pitch would be raised or lowered a half step for coloration. Only two signs were used in musica ficta; one to indicate when the pitch is raised and the other when the pitch is lowered. The natural sign came into use at about the time when a new tuning system was being developed around the late fifteenth century.