I'm a noob to music theory so please excuse this question if it is stupid. What is the rule of thumb in naming a sharp/flat note. For example: regarding a half step up from A - when does one use A# and when does one use Bb? Does it boil down to what is more popular or is there a music-theory-rule for this?

thanks in advance,
A# and Bb are called enharmonics. You can use either.
Quote by tattyreagh
Isn't it in the US Constitution that you can defecate on any carpets you like. If not then America is not the land of the free.

Quote by vulgarmachine
I like to stuff pizza crust up my bum.
Thanks. I should have added that I've witnessed some being used more than others - like C# rather than Db.
You can use either depending on what key signature has been defined. Major scales are already defined as having either sharps or flats. For example the B scale will have that A#in it but the F scale has a Bb instead. It honestly all depends on the key signature.
The only time it makes a difference what you call an enharmonic is when talking in terms of scales or intervals. If you need to find a fifth above C, it is G. A flat 5 is Gb, not A#. This is because you are flatting the 5, G.

Another instance would be when referring to key signatures. Written out on staff paper, Db major looks a lot different than C# major.

But in most contexts enharmonics can be referred to by either name. But as already said, some are more commonly stated than others. Neither are wrong, but if you ever play with horn players, you'll get along with them better if you make them play in Bb rather than A#.
Phish Phan
If there's a jam out there, I'm probably listening to it.

Check out the Bodatious Banana Extravaganza: http://myspace.com/bbeboston
Every diatonic scale has to use each of the 7 letters to name the scale tones, so you can't have an Ab and A# in the same scale, even though you would be technically correct. You could also not have a G# and a Bb and miss the A completely. You have to see what note comes before and after it in the scale your using to decide whether to use a sharp or flat.
Last edited by cal1fub3ralle5 at Jul 20, 2010,
Thanks everyone for the fast and helpful replies. Looks like I have some Music Theory to catch up on :-)