When an accidental is used to contradict the key signature it remains in effect until the next measure. In some non-tonal music written without key signatures, accidentals only apply to the one note. It is often up to the musician to figure out the convention of a score.
When used on a note they appear as shown in front of the note they affect, but when that note is named, the letter name appears first. Why?  Because a flat-B is a note that is out-of-tune!

OK, there are seven letter names per octave, yet we divide the octave into twelve equal parts. This means there are five unlettered notes in between. The extra five notes are represented by accidentals.
On the piano keyboard this can be clearly seen in the repeating pattern of five black notes for every seven white notes.
It is very important to know there is no accidental between
E and F, nor B and C.
This chart shows the accidentals that may appear with a note. Only sharps and flats are used in key signatures.

The pitch quiz will test you on identifying notes with all of these accidentals.

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