Example: if I was going to write an A# major scale key signature would I use the double sharp symbol in the key sig.?

Don't confuse yourself unduly, enharmonic notes are there to discuss function, primarily. If you travel around the Circle of Fifths (which is where and how you build key signatures) you don't see A#. The key signature you're looking for is Bb major, which has two flats.

Yeah, I understand all that stuff. It's an assignment for school. I'm not writing out a chart in A# or anything like that.

Since there can only be one of each "named" note in a major scale, you would use the double sharp

i.e. A#, B#, C##, D#, E#, F##, G##, A#

Mike

Well, you wouldn't write a piece in A#, but I guess it's conceivable that a piece might modulate to A# for a bit, requiring a key sig with double sharps.

Depends on how the modulation is prepared. I can't think offhand of an example of it, but I won't say "impossible." But a "Beach Boys modulation" (up a half step) would just be notated in Bb, as there's no preparation for that kind of modulation.

Actually, I think the correct notation for double-sharp is "×"

…so it would look something like

A# B# C× D# E# F× G× A#

- Wil

Actually, I think the correct notation for double-sharp is "×"

…so it would look something like

A# B# C× D# E# F× G× A#

- Wil

Yeah and of course the sharps would come before the note