When I have a client who wants to learn about music theory, I make sure we spend however many sessions it takes so that they understand the essentials of chord names, and don’t ever get caught using that dreaded term "Dominant 7th chord".
The most confusing part of chord naming is understanding the use of "7".There is no chord type called a 'dominant 7th'. It's just "7" or 7th". The term dominant 7, which derived from misuse of classical music terminology, refers to taking the V chord in a scale (the major based on the 5th degree of the scale) and making it a 7th chord. But let's back up to basics of chord naming and get there more clearly (all examples below refer to the key of C).
The names of chord are based on the distance (interval) between the root of the chord, and each of the other notes in the chord (i.e. the name of the chord tells you what notes are in the chord).
If the root of the chord is C, the next highest note is usually a third, or the note "E", the third note of the C scale. If the interval is a major 3 (2 steps, or 4 guitar frets, C-E), then the chord is major. We don’t have to say "major" in the chord name, as that is understood by default. If the interval is a minor 3rd (3 half-steps or 3 guitar frets, C-Eb), then the chord is a minor chord, and we use the word minor in the name of the chord – Thus, the word MINOR in a chord name always refers to the status of the 3rd. If the 3rd is moved one half-step (fret) in the other direction, the interval is now a perfect 4th (C-F), and the chord is called Suspended or sus4. The third is changed to the 4th. "Sus" in a chord name refers to changing the 3rd to the 4th.
The 5th in a chord is usually going to be a Perfect 5th or P5 (C-G) and if so, is not mentioned in the chord name. Like the major 3rd, it is the default (Thus a major chord - Root, Major 3rd, P5th) is just called by the letter name e,g,. "C". If we mess with the 5th, and move it down one fret, it is now a diminished 5th, and the chord name says (-5). If we push the 5th one fret higher, it is now an augmented 5th, and the chord name says "aug" or "+" . Thus, the chord C+ would mean C-E-G#, and the name C-5 would mean C-E-Gb.
Now, this brings us to the confusing 7. If we have a normal major chord, and add to it the note that is the normal 7th degree of the scale, the interval is a Major 7th and the chord is called maj7 or M7. CM7 = C-E-G-B. The major7th interval is always designated with "maj" or "M" If you reduce the M7th interval by one fret (e.g. from B to Bb) it is now a minor 7th interval. BUT, the word "minor" in a chord name is reserved to refer to the 3rd So we can't use the word "minor" to refer to adding a minor 7th interval to a chord. So what do we do? WE USE JUST THE NUMBER 7. Thus a 7 or 7th chord is root-Major 3rd-Perfect 5th-Minor 7th, or C-E-G-Bb. It has nothing to do with whether the chord is a dominant chord (built on the 5th degree of the scale) or not.
In the key of C, "G" is the 5th, so a G7 is the 7th of the dominant chord, but just as easily, F is the 4th, so an F7 chord in the key of C is the 7th of the Sub-Dominant, but no one ever says "F Sub-Dominant 7". D is the 2nd and a D7 is the 7th of the Super-Tonic but no one ever says D SuperTonic 7. It makes no sense, and is just a perpetuated misunderstanding of classical music terminology. (1 = Tonic, 2= Supertonic, 3=Mediant 4=Subdominant 5=Dominant, 6=Submediant, 7=Leading tone, 8=Octave)
Now what happens if you have a minor 3rd in a chord AND a minor 7th (C-Eb-G-Bb)? By the rules above, the minor 3rd is noted by the word 'minor', and the minor 7 by just the number 7, so you have Cm7. What if you have a minor 3rd and a major 7
(C-Eb-G-B) ? It is a Cm(M7). That name tells you every note in the chord. Similarly if you messed with the 5th, and had C-Eb-Gb-B you'd have Cm-5(M7).
You've probably seen the chord name "dim" or diminished. This means that EVERY interval in the chord is reduced one half-step. Thus a C7 (C-E-G-Bb) becomes
C-Eb-Gb-A (technically, the "A" is Bbb) and is called Cdim. In classical music, a diminished chord would not typically include the diminished 7th. Just the Root, m3rd, and dim 5th.
Other points to remember: Just a number after a chord name means that the chord includes every third interval from the root to that number, i.e "9" means Root, 3rd, 5th, 7th, 9th. An 11th chord means Root, 3rd, 5th, 7th, 9th, 11th. Depending on whether each of those intervals is raised or lowered, you would adjust the chord name accordingly.
C-Eb-G-Bb-D is root, m3, P5, m7, and maj9, and the chord name is Cm9. The "m" means the 3rd, and the 9 means both the 7th (NOTE that the "7th means the minor 7th, Bb instead of B), and the major 9. If instead you had C-E-G-B-D, it is now Cmaj9, as the "maj" indicates a major 7 interval, instead of a minor 7.
Last point: We commonly see chord names such as Cadd9. This means exactly what it says. It's a major chord (C-E-G) to which we ADD the major 9, which is D. (D is 9 because it is nine scale degrees above the root i.e., the octave C-to –C is 8 (C on the A-string to C on the B-string) plus one step to D is 9, which is why we stick our little finger on the 3rd fret of the B string while fingering a regular C to make Cadd9. Same concept on the keyboard.
There are many more chord names and variations, but the principles are all the same. The name of a chord, based on the distances from the root to each not in the chord, should tell you all the notes in that chord. – And there is NO SUCH CHORD TYPE AS A DOMINANT 7!
For more chord, scale, and rhythm theory, go to my book website, http://www.songcrafterscoloringbook.com
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