^ Good stuff. I wrote out a fair bit of similar information before I saw the above post, so I'll post it anyway. Wall of text, I SUMMON THEE!
Here is my guide to chord symbols:
Just a letter (capital) : Major triad (1 3 5) - ex. G = G B D, F# = F# A# C#, B♭ = B♭ D F
m, mi, min, - : Minor triad (1 ♭3 5) - ex. Gm = G B♭ D, F#- = F# A C#, B♭min = B♭ D♭ F
°, dim : Diminished triad (1 ♭3 ♭5) - ex. G° = G B♭ D♭, F#dim = F# A C, B♭° = B♭ D♭ F♭
+, aug : Augmented triad (1 3 #5) - ex. G+ = G B D#, F#+ = F# A# Cx ("x" means double sharp), B♭aug = B♭ D F#
Once you have the triad symbols, every other chord symbol is merely a matter of attaching something extra on the end. The conventions for labelling chords are very loose in practice, so I try to structure my system in modular units that eliminate ambiguity. Keeping the triad qualities in mind, let's move on to sevenths.
A seventh may be added to any triad as its own distinct quality. There are three kinds of sevenths:
∆, maj7, Ma7 : Major seventh, a half-step smaller than an octave. ex. D-C#, B♭-A, C-B
7 : Minor seventh, a whole step smaller than an octave. ex. D-C, B♭-A♭, C-B♭
°7, dim7 : Diminished seventh, an augmented second (the same space as a minor third, three half steps) smaller than an octave. ex. D- C♭, B♭-A♭♭, C-B♭♭
Leaving the diminished seventh alone for now, any of those can be attached to a triad to create a seventh chord. All you do is lop the seventh on the end of the triad symbol.
E♭maj7 = E♭ triad plus a major seventh from the root of the chord (D) : E♭ G B♭ D
G∆ = G triad plus a major seventh from the root (F#). G B D F#
Em∆ - Em triad plus a major seventh from the root. E G B D#
Cmi/maj7 - Cm triad plus a major seventh from the root. C E♭G B. notice that I am separating the "mi" and "maj7" with a /. This is to make it more readable. I don't like extra characters, so "m∆" is my preferred label. On that note, "-" could easily be a smudge on the page, and a tiny little line does not seem very deliberate, so I discourage the use of "-" for minor triads. "m" works much better.
D+∆ - D+ triad plus a major seventh from the root. D F#A# C#
B♭+∆ - B♭+ triad plus a major seventh from the root. B♭D F# A
°∆ is rare by comparison.
A+7 - Augmented triad with minor seventh; A C# E# G
A7 - Major triad with minor seventh; A C# E G
Am7 - Minor triad with minor seventh; A C E G
Aø7 - Diminished triad with minor seventh; A C E♭ G. This is a weird one. Also called a half-diminshed seventh (or m7♭5, nomenclature that makes no sense to me on the basis that the triad is diminished and not some hobbled minor chord). You'd think to use "°7" for consistency's sake, but we allow the exception for…
A°7 - A diminished triad with a diminished seventh; A C E♭ G♭. Also called the "fully diminished seventh chord", or simply "diminished seventh chord". This is the only instance where we use a diminished seventh in a chord.
After you have a seventh in the chord, any other tone you add is perceived as an "extension". We build chords in thirds, yielding odd-numbered chord tones up to 13, so the entire maxed out structure of a tertian chord looks like this:
1 3 5 7 9 11 13
All the red stuff is extensions. In order to have extensions you must have a seventh, otherwise it's something else. Furthermore, by having an extension in the chord symbol, it is assumed that all lower extensions are possibly in the chord. Therefore, "G13" assumes a G chord with a seventh of some sort (a minor seventh, actually), a 13 as the highest extension, and an 11and a 9 as the lower extensions. "G11" indicates the same exact thing, but only going up to 11 (i.e. not 13). "G9" is the same, but with neither 13 nor 11.
Sevenths are implied in the chord symbol in the same manner as they were when we looked at sevenths.
All of these have a major seventh: ∆, ∆9, ∆11, ∆13 (or maj7, maj9, maj11, maj13)
All of these have a minor seventh: 7, 9, 11, 13; also ø7, ø9, ø11 and ø13 for diminished triads
All of these have a diminished seventh: °7, °9, °11, °13 (though °13 is redundant; I am merely demonstrating the principle)
It is also assumed that, unless otherwise indicated, these numbers represent major and perfect intervals. So, A13 implies an A major triad with a minor seventh and a major thirteenth, perfect eleventh, and major ninth.
A13 = A C# E G B D F#
Minor, diminished, and augmented extensions are introduced with what we call "alterations". These are placed in parentheses to the right of the chord symbol in order from highest to lowest.
A13(#11) = A C# E G B D# F
If the highest extension is altered, then the next highest natural extension is listed in the first part of the chord symbol, followed by the alterations.
A11(♭13) = A C# E G B D F
A9(♭13,#11) = A C# E G B D# F
A7(♭13,#11,♭9) = A C# E G B♭ D# F
Some extended chords make use of split chord members, meaning one of the members of the chord has at least two qualities. An example:
E♭7(#9,♭9) = E♭ G B♭ D♭ F♭ F#
Notice the larger interval comes before the smaller interval in our alteration parentheses.
Normally, we want to avoid having two of the same letter note name, but situations such as these are unavoidable. For that chord, you can expect to see all sorts of enharmonic respelling to convey what's going on.
Next comes what I consider miscellany. I have given extensive coverage to tertian structures (triads, seventh chords, extended chords), which is the meat and potatoes of chord vocabulary. Other chords are not formed by the standard method of stacking thirds, and so require some explanation.
Some chords contain adde members. That is to say that you start with a triad, then throw something else into the mix. Added members are parenthesized.
C(add9) = C E G D
C(add2) = C E G D
Conventions for these chords are fast and loose. The above two chords have the same notes, but different names: one label prefers the simple interval (2) and the other prefers the conpound interval. Some practicioners will claim that an added ninth and an added second differ by where the added member is in relation to the other notes in the chord.
This is a common added member chord:
F6 = F A C D
This should technically be "F(add6)", but a century of looking the other way has made "F6" an acceptable notation. Nobody will know sht you mean by "F2," though, and "F9" is something completely different. A potential variant:
Fm6 = F A♭C D
Mind you, F°6 is not the chord that symbol would have us think it is.
F°6 = F A♭ C♭ D
F°7 = F A♭ C♭ E♭♭
Added members are sometimes combined in a chord.
F(add6,add9) = F A C D G
more commonly spelled with the shorthand
F6/9 = F A C D G
Alterations are also possible.
Fm(add#4) = F A B C
There are a few chord members that can never be added. These are members of the triad and the seventh chord. "C(add3)" makes no sense because C already implies C E G. "C(add7)" also does not exist, as C E G B is C∆. Also on that note, "C∆(add6)" does not exist. The presence of a seventh turns that sixth into a thirteenth; properly, it is "C∆13".
Finally, there are substituted chord members. These are often called by the name, "suspended chords" or "sus chords", though suspension is in reality a melodic procedure that requires preparation and is usually followed by resolution, not a static harmonic idea. Nonetheless, I think I am the only one who writes "E7(sub4)" where most would write "E7(sus4)". Sus chords omit the third of a chord in favor of a second or a fourth. These are different from added tone chords. Note:
E(add2) = E F# G# B
E(sus2) = E F# B
Added tone chords contain all of the normal members, and then some. Sus/sub chords have another tone in place of a regular member.