Basic theory question

[chadefa1]

Well, I know I should buy a theory book  . And I have! I ordered the Turek, but haven't received it yet  ...
So, in the meantime, I will ask my (stupid) question:

I have here the score of Bach's BWV 939 Prelude. In bars 9, 12 and 14 appear natural signs on an F, an F and a B respectively. Now my question is: why is there a need for a natural sign here, given that:
- the key signature doesn't specify that these should be sharp
- there are no previous accidentals in the bar that would require "corrections"

So, I thought: mm, maybe they only put one accidental  and from that accidental onward,  you should assume that this note is sharp of flat. But then how come the sharp on the F of bar 5 is repeated on bar 8??

I must be missing something very simple and will be ashamed when I see the answer, but I'd still appreciate to hear the answer 

best regards,
Thomas

[xvimbi]

Helpful editors usually put a natural sign in front of a note if the previous instance of that note had a sharp or a flat. That previous instance can be several measures away, and it can be in a different octave or even in a different staff. Theoretically, an accidental applies only to notes on the same line, but in practically all scores I have seen, accidentals apply to all the same notes in a staff, no matter what octave. So, if you have an F sharp on the top line of the staff, and the next F natural is two measures away in the bottom space of the staff, most editors will add a natural sign. Likewise, if the next F natural is in the other staff, most editors will also put a natural sign.

Looking at measure 9, there are F sharps in the previous measure in both staffs.
For measure 12, there is an F sharp in the previous measure in the top staff.
For measure 14, there is a Bb in the previous measure.
I don't know why there would be a naturalized B in measure 12.

Also, sometimes, accidentals are simply there to remind the performer; it's a courtesy.

[chadefa1]

Thanks a lot Xvimbi. As usual great help!
I thought that all accidentals "dropped" at the end of each bar. So, if there is an A sharp in the first measure, I should play all As sharp after that one, until I see a natural sign?

There is no naturalized B in bar 12, but in bar 14, because there was a flat in bar 13.

Thanks again.
best,
Thomas

[xvimbi]

Thanks a lot Xvimbi. As usual great help!
I thought that all accidentals "dropped" at the end of each bar. So, if there is an A sharp in the first measure, I should play all As sharp after that one, until I see a natural sign?

No. It is correct that accidentals get "dropped" at the end of each measure, so you should not do what you just described. It is only with these "courtesy accidentals" that they may refer to a note that has occurred in a previous measure.

There is no naturalized B in bar 12, but in bar 14, because there was a flat in bar 13.

I misread your post

[chadefa1]

Very good, thanks!
I find the "courtesy" accidentals more confusing than anything, but well... At least I know what to play.

Thank you

[xvimbi]

Very good, thanks!
I find the "courtesy" accidentals more confusing than anything, but well... At least I know what to play.

They are less confusing and more helpful for sightreading.

[cadenz]

you shouldn't find these "courtesy accidentals" confusing. you need to realize, they don't -have- to be there, they're just there to remind you to go back to the dropped accidental form of the note. you may find the idea of when they're used and when they're not used confusing, well this is because they're not necessary, so they're mostly just used when a certain note has been repeatedly affected by accidentals for a while to remind you that you need to go back to its un-affected form.