I understand that a C7 chord contains a flattened 7th note in the C major scale, but what specifically makes this a dominant 7th chord? To me there is nothing 'dominant' about taking a note out of the major scale and flattening it. Can anyone elaborate on why this is the case?
It is so called because B♭ is the 7th note of the C dominant scale (also known as the Mixolydian scale).
The 5th is known as the dominant, because it is the "most important" interval (among other things, it's the first harmonic other than the octave).
However, try to forget the "English" meaning of the word "dominant" -- otherwise you might expect C to be the "dominant" of C. It's not. In C, C is the "tonic". G, the 5th is the "dominant". Other intervals have names like "subdominant" (4th) and "mediant" (3rd) , which at least don't confuse you by having instinctive meanings.
5th relative to C is G. So since the C major scale is C,D,E,F,G,A,B, then the G dominant scale is G,A,B,C,D,E,F
... and the chord of G dominant 7th is the triad plus the 7th from that scale - G,B,D,F
The C dominant scale contains the same notes as F major, since C is F's 5th:
- F major: F,G,A,B♭,C,D,E
- C dominant: C,D,E,F,G,A,B♭.
So the chord of C dominant 7th is C,E,G,B♭.
Since in most Western music, major is the "default" scale, we might expect "C7" to denote C major 7th - C,E,G,B.
However in pop/rock/etc., the major 7th is relatively rare, and dominant 7ths are very common. Hence people organically settled on the convention of using "C7" to denote the dominant 7th and "Cmaj7" to denote a major 7th.
tl;dr: Early on in common practice music the V7 chord construction was frequently encountered (note I mean the dominant seventh of the current key); over time,the name "dominant seventh" came to be applied to any major triad with a flattened seventh degree, even when you're not dealing with the dominant of the current tonality.
C7 is, in the most literal sense, the dominant triad, with an added seventh, in the key of F major. One of the most common uses of the dominant, fifth (V), chord is in a perfect cadence, V->I. Although this is done with just triads, including the seventh of the dominant chord reinforces the finality of the cadence. Over time, the designation of these "dominant seventh" chords expanded to any (major) chord involving the flattened seventh degree.
For many reasons, I hate using the term "Dominant seventh chord." A more proper term is the "Major-Minor seventh chord" as it more accurately describes the intervals of the chord. The chord is a major triad built on C with a minor seventh appended to the top. Spelled out, this would be "C E G Bb." Why is it "dominant?" In typical tonal harmony, the key of F's dominant counterpart is C. The dominant triad of F is therefore the C major triad, "C E G." However, over the course of musical history, things have been stuck to the tops of triads, so to speak, and it happens first to dominant harmony. Adding this seventh to the dominant triad in a key gives the dominant more weight. So, "C E G Bb" is a Dominant seventh chord because its function in the key of F, where this chordal structure would have originiated, is dominant harmony.
Adding to the above great answers, 'dominant' means powerful, pushy and as such the dominant chord in a series 'pushes' the sequence towards the tonic. Look at any piece of music and there is far more chance of a chord being preceded by its dominant than by any other. E.g. G goes to C; A goes to D; B goes to E, etc. This happens with minors as well. It's thought that the leading note from the dominant chord feels like it needs to resolve on the tonic note, as in the B of (dominant G) moving a semitone to C.When the dominant has the 7th added (in this case F), then the F wants to resolve down to the 3rd (E) of the tonic.
In the key of C, if you play only the B and F notes out of the G7 chord, it sounds 'odd'. It's actually the flat fifth used in Blues. After this, play C and E just next to those notes, and the tension is released.The 'dominant ' bit is in the sound of the first 'chord' pushing to its resolution.
When making basic chords (triads), alternate notes are used, e.g. I-III-V,etc.This gives 7 different chords for each key. Adding the next alternate note, I-III-V-VII gives a tetrachord, 7 of which become available,built off the 7 chords. This progression continues with IX, then XI, etc, to be 9ths and 11ths.Depending on ,in particular, the III and the VII, the chord will be called maj., min., or dominant. Thus C-E-G-B-D becomes maj.9th, whereas C-E-G-Bb-D is dominant 9th.
So, dominant 7th =I-III-V-bVII. Major 7th = I-III-V-VII, and minor 7th = I- bIII- V-bVII. The three 7th chords in general Western use.
Another, perhaps easier way of thinking of this, is to just recognise that with a diatonic set of notes, such as the notes in C major, there are 7 scales you can play.
In the list below, I include the degree as a roman numeral, the name of the mode starting on that note, the name of the scale degree, the notes if played using C major notes, the interval of each step (T=tone, S=semitone)
I - C ionian - tonic - CDEFGABC - TTsTTTs II - D dorian - supertonic - DEFGABCD - TsTTTsT III - E phrygian - mediant - EFGABCDE - sTTTsTT IV - F lydian - subdominant - FGABCDEF - TTTsTTs V - G mixolydian - dominant - GABCDEFG - TTsTTsT VI - A aeolian - submediant - ABCDEFGA - TsTTsTT VII - B locrian - subtonic - BCDEFGAB - sTTsTTT
"G dominant 7th" is the triad plus the 7th note of the G dominant, aka G mixolodian scale.
Now, if the notes of C Ionian (the white notes on a piano) are the notes of G Dominant, what Ionian scale corresponds to C Dominant? It's the same interval. To go down from G to C is 7 semitones. Down 7 semitones from C is F, and F Ionian contains Bb.
To quote Wikipedia :
The name comes from the fact that the flat seventh occurs naturally in the chord built upon the dominant
So for example, if you take the C major scale (C, D, E, F, G, A, B, C), G is the dominant degree of C.
We then can build a seventh chord for G based on C's major scale, starting with G, which give us G, B, D, F. F is thus the "dominant 7th" of G, but also the minor 7th.
Because it's 'in key' (no accidentals) when rooted on the dominant of the key..... ie. In the key of C (all white notes) G7 uses all white notes..... C7 and F7 both have the seventh as an accidental (black note in C). So to stay in key you have major sevenths built on the first and fouth chords (root and subdominant), and dominant seventh on the 5th chord (dominant)...
I think it's far simpler than most of these answers, though many allude to it. First, you have to understand modes. In the mixolydian, or "dominant", mode of any diatonic scale, the seventh is flat, hence, any chord featuring a flattened 7th is referred to as a "dominant" 7th. So, basically, what slim and many other folks said, but made simple, and without trying to explain the concept of "modes" in a paragraph.
Actually, I know C7 as a seventh chord, period. Now a seventh chord in a regular cadenza occurs on the dominant (which is a fifth above the tonic). C7 is the dominant seventh chord for an F major tonic, used in an F major cadenza.
Calling it a dominant seventh chord outside of its harmonic relation to a tonic is not actually correct or useful.
But a lot things not actually correct or useful have become sort of common or even customary. Particularly if they roll off the tongue as nicely as "dominant seventh".