Accidentals are musical signs which change the pitch of a note. The two main ones you may have already encountered in your musical studies are the sharp and the flat. Below is what a sharp and flat look like in music.
The sharp raises the note by a half step. The flat does the opposite. It lowers the pitch by half a step. Sharps and flats are usually the black keys on a piano (see the diagram below).
The Natural
There are times in music that you will need to cancel a sharp or flat. For that, we use a sign called the natural. The natural restores a note to its original pitch. In the example below, note A sharp (the first note in the music) is restored to its original (note A) by the placement of the natural in front of it.
Accidentals last only until the end of the measure in which they appear. In the example below, note C sharp (in bar 1) is cancelled by the bar line. This means that note C in bar 2 (beat 1) is no longer affected by the sharp.
Accidentals appearing on a bar, affect every note on that space or line for the remaining of the bar unless cancelled by a natural. In the example below, note B flat in beat one, affects the B on beat 3 as it is on the same line. In the second bar, the B flat is cancelled by the natural.
As mentioned before, accidentals on a line or space, only affect the note on that line or space. Therefore, in the example below (bar 1), the first F is an F sharp but the second F is an F natural.
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When the sharp sign is place in front of two notes, for example f, and a , do I sharpen both notes?
Yes that is correct. If you want to add accidentals in front of two notes, you place sharps (or flats if that is what is required) in front of each of the notes.
So only for that line? different octaves from the original accidentaled note are not affected? Also what about grace notes is it the same thing or different? ie do they change the whole bar?
Good questions Fred. Yes, notes on different octaves are affected by accidentals in that bar. Regarding grace notes: they too are affected by accidentals. Hope this clarifies it.