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In the staff, would one write enharmonic notes with # or b? Does it matter which you'd use and why?

For example: In the key of C Major, would it be better to write this passage with an A#, as it is, or Bb?

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In short, what rules govern this issue?

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Capital Roman numerals depict major and lower case depict minor.Playing this from the dots, to me now sounds like Amin. moving through C dom.7th to Dmin. If this is what the composer intended, the A# needs to be Bb, the flattened 7th of C. –  Tim Oct 11 '13 at 18:15
You were right, it was actually vi. Is it ambiguous now? –  Jovito Oct 11 '13 at 18:20
That's good, my last comment still stands. –  Tim Oct 11 '13 at 18:26

4 Answers 4

up vote 30 down vote accepted

There are a few general rules.

  • Most accidentals should be of the type found in the key signature. For example, in G Major, use G# -- not Ab. In F# major, use A# instead of Bb.
  • If the accidental is in a chromatic scale, use sharps ascending and flats descending
  • In any other scale, use the accidental that typically goes with the scale. For example, Bb and Eb in a Bb major scale instead of A# and D#.
  • In a chord, use the normal accidentals so it is recognizable. For example, Eb in a Cm chord instead of D#. In the example from the question, prefer A#. An A# dim chord is usually spelled A# C E. A Bb dim chord is usually spelled Bb Dbb Fb, which is a lot more unusual than an A# dim chord.A musician will bore easily recognize A# dim.
  • Of the enharmonics, prefer C#, Eb, F#, Ab, and Bb when no other rules apply. For example, in C major, prefer F# instead of Gb. This is assuming no other rules apply and neither accidental is easier to read than the other.

The reason for these enharmonics is the number of key signatures they are naturally found in. C# is found in 6 key signatures, while Db is only found in four. So, C# is the primary name for that enharmonic. D# is found in four, while Eb is found in 6. You can find a complete list here.

However, if one accidental makes a passage a lot easier to read, prefer that one over the one specified in these rules. For example, E D# E would be preferred to E Eb E♮.

Basically, these are general guidelines and have plenty of exceptions.

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Great answer here - just want to add (at the risk of confusing someone) it's also important to keep in mind the nature of the instrument you're writing for (saxes are more used to seeing sharps / brass are more used to seeing flats.) A very small but important point. –  jjmusicnotes Oct 11 '13 at 0:35
Also, if you're writing music more on the edges of tonality, you want to avoid mixing flats and sharps within the same chord or measure. Stick to one type and it'll be easier for the player to read. –  NReilingh Oct 11 '13 at 3:17
Great answer. The basic philosophy is that you want to use the one that will be easiest for the musician to read. These rules should help you get there. –  Ben Miller Oct 11 '13 at 3:43
@Tim - because of the transposition, saxes, though pitched in Bb and Eb, tend to be put into sharp keys - especially Eb saxes. Brass players are comfortable reading flats from the band world where Bb, Eb, and Ab are the "standard" band keys; even Db is quite comfortable as well. If you want to hear something scary, ask a beginning band to play in E major. –  jjmusicnotes Oct 11 '13 at 14:07
Another thing to note is on chromatic instruments which have parts associated with particular pitches (e.g. organ pipes and reeds, piano strings, actuating levers, etc.) it is common to always use the "sharp" forms of pitches to identify them. For example, if the bottom note on a pipe organ rank is C, the fourth pipe from the bottom would be described as D# (rather than Eb). –  supercat Oct 18 '13 at 17:05

Truth is, it doesn't really matter... but in general, you should write it so it is easily understood. Some helpful tips for making it easy to understand are:

If you are going up, it's easier to read a sharp (C up to D# instead of C up to Eb)

If you are going down, it's easier to read a flat (C down to Bb instead of C down to A#)

Use accidentals of the same key (key of G uses sharps, key of F uses flats)

Avoid mixing sharps and flats together (A# with Gb is awkward to read)

No unison enharmonics (don't write a B# and a C♮ in the same measure)

No reversed enharmonics (don't write a B# and a Cb in the same measure)

No inversed enharmonics (don't write a Cb and a C♮ in the same measure)

No contradicting accidentals (don't write Db and D# in the same measure)

Use the least amount of accidentals (reading A#, B, A# is easier than reading Bb, Cb, Bb)

Put every note on a letter if you can (Instead of writing C, B, Bb, G, Gb, F, Eb, C; it's better to write it as C, B, A#, G, F#, E#, D#, C because each note is assigned to a letter)

Follow music theory if it applies (sharps for augmented steps, flats for diminished steps)

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In addition to American Luke's answer. Each key has its own key signature, as in, Gmaj. =F# only. Gmin. has Bb and Eb. So, using your example, in key of F#, the A notes would already be sharpened by virtue of the key signature, which will contain a sharp sign on the A space (treble clef).

An important factor is that once you've started a tune using one or the other, it USUALLY makes sense (and is often easier to read) if you continue to use the same. However, technically this is not always the case. Take the tune in,say, G maj. using one # all through. To use a chord of Cmin., the E will need to be flattened, 'cos that's exactly what has happened to it. It wasn't a D that was sharpened, so in a 'sharp' key, one would have to use a flat for that note.

Another point to help you is that in ANY key, there will be a note (from its scale) for EACH letter of the musical alphabet. Thus, in spelling, for example, Bb, the notes would be Bb, C, D, Eb, F, G, A, Bb. Thus, the Eb couldn't possibly be D# as there is already a D there.Otherwise, writing on the stave would become a mess with naturals cancelling bs and #s all over.

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Please bear in mind this is an answer to the UNEDITED original question, not particularly an answer to the EDITED one. –  Tim Oct 11 '13 at 16:11

As the OP's question has now gone on a different tilt, here is a new answer...

Taking your point of writing a C note in the key of F# : A## (or Ax) would give the enharmonic B, so wouldn't work.Cb is unnecessary, as this would make B also. The way to show C is to write C natural (can't find the qwerty key for this !), as it is a C note that you want played.If the note came from, say, an augmented 4th interval such as F# to B,augmented, the note would be shown as B#. Hope this answers the 'new' question.

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