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 double sharp indicates
that the note is raised two chromatic steps.
A double flat indicates
that the note is lowered two chromatic steps.
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.