Ask HN: When in the key of F-Major would you use C# or Db in notation?
Obviously nothing native to the key, but various borrowed chords and what-not, such as: a V-I tritone substitution of Gb to F would include a Db; augmented 6th chords, such as German or Italian 6ths, built off of Db directly; a borrowed dominant leading to Dm (A7) would include a C#.<p>EDIT to add: Also altered dominant chords, such as building a diatonic C7 into a C7(b9) would include a Db. Getting further afield, if you borrowed a Bbm into the key of F, you might approach the Bbm with a dominant 7(#5) chord, yielding an F7(#5) with a C#.<p>Lots of fun possibilities! :-)