For the chord C7 (specifically!), why do you write Bb and not A#?

Question

For the chord C7 (specifically!), why do you write B♭ and not A♯?

Answer 1

The "C" part of the chord's name tells you that the chord's root (or 1st degree) is C. This means that the letter part of the name of its 7th degree is B. (Counting from the 1st to the 7th degree: C D E F G A B.) This applies to any sort of 7th-chord whose root is C. C7 means a C major triad with a minor 7th, and, as Tim pointed out, the note a minor 7th above C is B♭.

Answer 2

Don't know if this tells you something you didn't already know, but here goes:

"C7" designates a C major chord with an added seventh interval. Here a "minor seventh", it would become a "major seventh" if you played the B natural with the C major, widening the interval by a half-step.

You will find that the notes C and B, are seven steps apart, counting "1" on the C. Even if a B flat (B♭) is at issue here, the number assigned to an interval (seventh/third/fifth/etc.) comes from counting the letters, corresponding to the white keys on the piano, in order: "C D E F G A B".

Why call it a C7 chord and not a "C augmented 6th", though — which would warrant calling the note A♯ instead of B♭? (six letters counting from C to A. (To be sure: These chords, built as simply as possible, are all the exact same keys on the piano. Regardless of what we call them.)

Because: Music in the Western world has a fundamental practice of building chords in thirds on top of each other. C D E F G A B. Skipping one letter, as I have done in bold type here, gives the interval of a third every time, C to E, then E to G, and so on. Thus we arrive at a B — whether that is a natural B, or a flattened one — by taking the process of building a C major chord just one step further. This happens all the time, because of thirds being stacked onto eachother all the time to make chords. Therefore, basic three-note chords "with a seventh added" happens all the time, and is more theoretically fundamental than chords with added 6ths.

Answer 3

There is a formula for writing notes and intervals in the major scale as well as those that appear in any chord.

In any key the letter names of the notes of the major scale in order of appearance are a 7 character (or 8 if you include the octave), unbroken, sequence from the string...

{..., A, B, C, D, E, F, G, A, B, C, D, E, F, G, A, ...}

Appropriate sharps and flats need to be added to get the correct intervals {w, w, h, w, w, w, h} between consecutive notes. The naturally occurring steps are all w = whole with the exception of {B, C} and {E, F} which are each h = half.

The C major scale is {C, D, E, F, G, A, B, C}.

The A major scale letter names are {A, B, C, D, E, F, G, A} and to get the correct steps we need sharps --> {A, B, C#, D, E, F#, G#, A}. The accidentals do not change the letter names.

We also refer to the notes by degree, which is just an integer indexing the location of that note in the scale. For the major scale the degrees are always {1, 2, 3, 4, 5, 6, 7, 8}. So the 2nd degree of C is D while the 2nd degree of A is B. Degrees are universal in some sense.

The chords are constructed from consecutive 3rds, or every other note in the scale.

Some formulas for chords are as follows:

Major = (1, 3, 5)

Minor = (1, b3, 5)

Maj 6 = (1, 3, 5, 6)

7th = (1, 3, 5, b7)

etc.

Applying this to C one gets the following.

C Maj = (C, E, G)

C min = (C, Eb, G)

C maj 6 = (C, E, G, A)

C7 = (C, E, G, Bb)

Simply put B is the 7th degree of C and the formula mandates that this note be a flattened 7th, rather than a sharp 6th.

The reason you asked is probably because you are familiar with enharmonic notes. In the 12TET tuning system the frequency of an A# is identical to a Bb, in all keys. This is why equal temperament was invented. In other tuning systems these notes are not the same frequency. To ensure consistency across all spellings of a chord form we have adopted these conventions. A "7th" chord is (1, 3, 5, b7) and should be spelled as such in any and all keys. A Bb7 is (Bb, D, F, Ab) for example. A Gb7 would have an Fb even though this is also enharmonically equivalent to an E. I used to get points off on music theory quizzes in high school because I'd just through down any whacked out combination of enharmonic names to annoy my teacher.

Extensions of the dominant chord include the 13th, which will have the 6th degree in it. So a C13 chord would be (1, 3, 5, b7, 9, 11, 13), or reduced form might be (1, 3, b7, 9, 13) = (C, E, G, Bb, D, A). Having 2 A's in this would be confusing in terms of function.

Answer 4

The chord C7 comes essentially from the key F. It's the dominant chord in that key. That key contains a B♭, it's in the key signature. There is no A♯ anywhere around, as the A notes in that key are A♮s.

EDIT: The C part is the name of the root. The 7th part refers to the interval that number represents. CDEFGAB. It's an interval of a 7th. But why isn't it CEGB, but CEGB♭? That's in the first part of my answer - it's not from key C - where it could be B, but in key F (where it's from), the 7th from C is B♭. Hopefully this makes more sense than explanations about minor seventh intervals.

EXTRA EDIT: the 'seventh' part of C7 must be a B note of some sort. It cannot be any other! CDEFGAB - 7. So in direct answer to 'why isn't it A♯?' That would make it a C6 of some sort. It's not - it's C7. Now, there are various different sorts of 7ths, the ones emanating from C (major) being the note B♮, the note B♭, and the note B♭♭. The first is called C major seventh, the second C (dominant) 7th, and the last C diminished seventh, all based on the C major triad.Not an A note in sight ! O.K., that diminished looks like an A note, and sounds like an A note, but, honestly, it's a B note - in name. B♭♭.

Spelling C& with an A♯ instead actually makes a C augmented sixth chord - which does sound remarkably similar - identical in 12tet in fact, but because of it being spelt with A♯, it will never be called C7.

Answer 5

It's because tonal harmony is tertian meaning the chords are built by stacking thirds.

If we look at the two alternate spelling, and root position versus inversions, it should be clearer why using the right enharmonic spelling makes a difference.

When the root is C and we stack up the thirds it's easy to see the first chord is a root position seventh chord, and when inverting the top tone it becomes a third inversion seventh chord...

If the top tones was enharmonically spelled A♯, then on staff the chord looks like a first inversion seventh chord. When the top tone is inverted the chord looks like a root position chord. I say "looks like", but in reality if you spell the chord with A♯, you actually are setting the root as A♯ and the chord's third as C in the tertian system of harmony.

That's really confusing if your intention is a C dominant seventh chord.

When the A♯ is in the bass (remember that's root position) we get a diminished fifth between the root and the fifth of the chord. Immediately we read this is some kind of diminished chord. Looking at all the intervals above the bass we have: d3, d5, d7 diminished third, diminished fifth, diminished seventh. A diminished seventh chord altered with a diminished third enharmonically equal to a dominant seventh chord?!?

The importance of the correct spelling becomes more apparent when you work with harmony in all 24 major and minor key with the full common practice vocabulary of seventh chords and typical chromatic chords.

Sometimes unusual enharmonic spelling are exploited for various reasons like a modulation or a chromatic passage, etc. But if the case is straight forward diatonic harmony like V7 I, then spell the chord in the standard way.

Simple V7 I in F major...

Enharmonic spelling for a chromatic augmented sixth chord to V in E minor...

Answer 6

The names and symbols of chords (at least in common practice music) carry harmonic information as well as indicating the notes actually played. Chord symbols are not a variant of tablature (and vice versa.) The symbol, C7, indicates the chord C-E-G-Bb as indicated in other answers. There is the hint that the Bb will move downward, likely to A.

There is a commonly used chord, C-E-G-A# which on keyboards or fretted instruments are the same notes. (For strings and voices and in theory, A# is not the same as Bb; they are called enharmonically equal to indicate the difference. The C-E-G-A# chord is a German Sixth (one of the Augmented Sixth chords) in the key of E.

The important point isn't the names but the resolution of the chords. The general rule is that lowered intervals tend to contract and expanded intervals tend to expand. Also, semitone movent is "stronger" (more noticeable by the audience?) than whole-tone movement. The C7 chord tends to resolve to some form of F chord; the E moves to F and the Bb moves to A. Other tones follow voice leading rules. (In 4-part harmony, dominant seventh to tonic movement isn't easy.) Thus the German Sixth resolves outward C-E-G-A# goes to B-E-G#-B (and then to B-D#-F#-B with another half-step movement in the E-D# move.)

The point is that a performer whether playing from music, memory, or just making things up as it goes, can tell from the name or symbol what is likely to come next. Traditional notation carries lots of information.

Answer 7

I had this confusion too for a long time, so I thought I might add my two cents and state what I haven't seen stated explicitly. That is, it is because of a convention brought about for historical reasons that are no longer relevant.

However, with modern music theory, the convention has actually turned out to be useful for other reasons. But it is nonetheless a historical accident.

The convention is this:

For all common 7 tone scales, it is a convention that we use all letters from A to G exactly once.

This is relevant because when we use numbers to denote chord extensions (such as the 7 in C7, or in Cmaj7), what we are saying is that we mean the nth note in some scale. So for your example of C7, it is assumed that we are using the dominant scale with root C, and that the 7 means the normal C major, but add the 7th note of the scale.

When we take into consideration the convention of using all the notes A to G exactly once, the dominant scale is spelled C D E F G A Bb; it is not spelled C D E F G A A#, since the latter would break the convention. Our chord is therefore C E G Bb, and not C E G A#.

But I find it important to note again that it is only convention.

But we keep the convention for two reasons

1. It is hard to break such a long standing and old convention

2. It has proved to be useful for communicating the context of the chord.

The second reason is more relevant than the first, so I will elaborate on that one.

As mentioned earlier, saying C7 brings with it a certain scale that the chord is assumed to be built on, that is, the dominant scale. This communicates immediately that the function of the chord is a dominant one, since there are no other modes with a major 3rd and a flat 7th. In other words, if you are reading a well written piece of sheet music, and you see C E G Bb, you can know that whoever composed the song is probably communicating to you that they mean to use this chord to lead into the next chord.

This means, that if you are, say, improvising, or coming up with your own arrangement of a song, you can easily come up with more ways to alter the chord to express yourself without sounding off. As long as you keep that function of leading into the next chord, you can substitute almost any other chord and it will probably sound okay. Any voice leading that sounds like it very strongly leads into the next one will probably sound fine.

If say, you saw C E G A# in a well written piece of sheet music, then you will assume that the composer means to communicate something different. This may communicate, for example, that they are building off of the regular C major scale, or maybe some more exotic scale like C D E F G A# B, (not sure what its called, but playing it sounds very bluesy, and not as random as you might think), in which the chord may actually have a tonic function, and thus, again, if you were arranging or improvising, you might be thinking more "tonically" when substituting or improvising over this chord (common in blues).

Of course, this is all fairly hypothetical, and you will often see a dominate 7th chord denote a chord with a tonic function as well (such as in a blues progression). So really, it is convention, albeit a somewhat useful one for communicating context and intent.

Answer 8

The seventh chord is built from the 1st, 3rd, 5th, and 7th step from the root note. That seventh interval has to be a minor seventh. In C7, this would be C D E F G A Bb. Huh???

I'll add a snippet of knowledge by starting with the importance (and difference) of quality and distance in intervals. An interval is just two notes, and when we go to measure, we have to think about how far they are apart, and which notes to use to write them down.

Which notes we use to write them down is the distance. For example, C to E will always a third, C to F will always be a fourth, and C to B will always be a seventh, no matter how many accidentals you attach to either notes. You find this by first converting everything to regular natural notes and counting what step the top note is in relation to the root one

Quality is a bit trickier. It evaluates what the descriptor for the distance. We can agree that C to Eb and C to E are both a third, but they are clearly NOT the same. In this case, you have to memorize the order the difference between minor and major (and perfect, diminished, and augmented, but let's not overcomplicate now) intervals. For a seventh to be minor seventh, it has to be five whole steps away from the root, and respectively for a major seventh it has to be 5.5 whole steps away

Back to C7. Since we know it's a seventh, we know that the two notes are C and some form of B. That's our distance. The B then has to be flatter because we want to make it a minor seventh to satisfy the quality of the chord. A#, though it would an enharmonic equivalent, would make it a augmented sixth, and that would be a completely different chord theoretically

Answer 9

Because C,E,G,A is C6 (and A# would be C +13) and not C7. (The 7th degree belonging to the C scale is B and not Bb)

C7 = V7 of F! It stands for the dominant 7th chord C,E,G,B♭ => so,ti,re,fa of the F keys.

Answer 10

Others have given good answers already. (1) There are seven letter names in a scale and each one can be either natural, sharp, or flat. (2) Chord names are aliases for stacks of thirds. But I'll add a practical "test" you can try, to get an intuitive grasp on what's happening with the C7.

Let's say you have this melody line that's played over a C major chord. The melody has the notes: C - B - A - G - C., meaning, 1st, 7th, 6th, 5th, 1st scale degree.

Original melody

Now you want to create a variation of that melody line so that it fits over a C7 chord - what do you do? Compare these alternative melody lines:

(alt. 1) C - B - A# - G - C

(alt. 2) C - Bb - A - G - C

If you play these, it should be immediately obvious that alternative 2 is the correct one, and it reveals the fact that the C7 chord makes the scale's B note flat. It does not make the A sharp.

When you say "A#", it means that your A is sharp. Your 6th scale degree is sharp. By adding the title you say something about your A. It's like saying "Mr. A" or "Mrs. A". You have one "A" slot in the 7-degree scale, and it can be only sharp or natural or flat, but only one of those at any given time.

(The general "wrongness" of alternative 1 can be debated, but if the idea is to modify the original C - B - A - G - C to suit changing the chord to C7, I'd say that alternative 2 can fairly well be called the correct one.)