Sometimes a tune departs momentarily from the key, necessitating the cancelling
of what is required by the key signature. This is done by putting the required signЧsharp,
flat, or naturalЧbeside the note in question.
A natural sign ( ) means Уnot sharp or flat,Ф so when we refer to A-natural we
simply mean plain A, not altered by a sharp or flat.
Accidentals affect all notes until the end of the measure, after which the change
is cancelled. The only exception is when a note is tied over, in which case it retains
its changed status. The examples below make these rules clear.