When to write a sharp or flat ?

[Roscoe East]

There are conventions galore that will determine which accidental is "correct" but the most important consideration imho is What Makes The Passage Easiest To Read/Understand By The Musician Who Has To Perform It?

E.g., most musicians are accustomed to seeing stepwise motion, or triadic arpeggios spelled out so that the implied chord shape is recognizeable...while there are some situations where the prevailing key signature, or direction of motion, might make a different spelling more "correct", it almost always yields more immediate results to temporarily violate convention to allow the performer to quickly recognize these patterns.

 

[Lynne Davis]

When deciding whether to use a sharp, flat, double sharp or double flat to alter the chord tones of any arpeggio, including extensions, the general rule is: you cannot change the letter name of the note. Alter it however you like, but the letter must remain the same.

 

[MalcolmAmos]

Just dug up something that helped me a time long ago.

You can construct the Major scale using this limerick.

For the order of all the sharps: See God Destroy All Earth By F#iery C#haos
C,G,D,A,E,B,F#,C#
And for the number of sharps: See Fat Cats Go Down Alleys Eating Birds
Put the two limericks under each other. See (C) has 0 #’s, G has the F#, D picks up the C# and keeps the F#. A picks up the G# and keeps the F#, C# & G#. Etc. etc. etc.

For the order of all the flats: Farmer Brown Eats Apple Dumplings Greasily Cooked
And for the number of flats: Farm boys eat all donuts girls cook
Put the two limericks under each other F has 1 flat the Bb, Bb is the next in order and picks up the Eb (next in order). Eb is the next in order and picks up the Ab and keeps the Bb, etc. etc. etc.

OK now write the Major scale using columns:
I ii iii IV V vi vii

Now take the Major scale you wrote and do the following to produce the Minor scale.

Major scale column vi becomes the minor i
Major scale column viidim becomes the minor iidim
Major scale column I becomes the Major III
Major scale column ii becomes the minor iv
Major scale column iii becomes the minor v
Major scale column IV becomes the Major VI
Major scale column V becomes the Major VII

Using those two limericks and the substitution of columns you can construct the Major and minor scales anytime and anywhere you like.

Or do it this way. Use the chromatic scale as your base.

C,C#/Db, D, D#/Eb, E, F, F#/Gb, G, G#/Ab, A, A#/Bb, B, C.
The single letters like C, D, E, F, G, A and B are the white keys of the piano. The double letters like C#/Db or D#/Eb are the black keys of the piano. Two names for one sound. They are the in between notes --- one up or one back ---

Now the interval step for the Major scale is WWHWWWH and the minor scale is WHWWHWW. I remember them best as phone numbers. Major is 221-2221 and minor is 212-2122.

Pick a letter. Lets start with D and make a D Major scale.

D, now move up two notes for E, move two more for F#, now move one for G, then two more for A, then two more for B, and now two more for C# then one for D.

The Major scales of C, G, D, A, E, B, F# and C# get the sharps and the F, Bb, Eb, Ab, Db, G, and Cb get the flats.

Have fun.

From the above I made myself a couple of charts and framed them. Making them helped me realize how sharps and flats get into scales. Having them framed lets me sneak a peak when ever I need to.