Hoo-boy... I'll give it a shot. I apologize if I start out too basic, but it's important that we're all coming from the same foundation. There's really a lot of music theory going on here:
A chord is made up of (at least) three notes. The standard form of a chord is called "root position". In root position, the lowest-pitched note in the chord is called the "root", and it's that root that gives the chord its name. It really is just like the root of a tree.
So a "C" chord has the note "C" as its root. So far, so good...
Now, if we're talking about a regular ol' major chord, the next note up is a third away from the root. The note after that is a fifth away from the root. That's the formula for all major chords. Root, third, fifth.
Ah, but what's a "third"? And what's a "fifth"?
Well, that's all based on the major scale. In the key of C, the major scale has these notes, in order from low to high:
C D E F G A B
The third note you play (which happens to be "E") is considered to be a "major third" away from the root (which is "C"). The fifth note you play (which is "G") is considered to be a "perfect fifth" away from the root ("C").
So a C major chord is made up of these notes: C (the root), E (the third), and G (the fifth).
Like I said, this is the standard formula. So this relationship to the root note is the same no matter what key you're in. If you're in the key of F, then you're dealing with the F major scale:
F G A Bb C D E
Guess what the notes are for an F major chord? Yup! It's F (the root), A (the third), and C (the fifth). Different notes, but the same parts of the corresponding scale.
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If a chord is called an "open fifth", that means that you leave out the third. The literally leaves things "open" between the root and the fifth. You might also hear this called a "power chord", especially in the world of rock guitar (where open fifth chords are used quite a bit).
So if a C major chord is C-E-G, what's a C open fifth? It's C-G. Just those two notes--root and fifth. No E (third) allowed!
A B major scale looks like this: B C# D# E F# G# A#
Do you see how B and F# really do make an "open fifth"? Eventually you'll get the hang of all the keys, and you'll be able to tell what note is a fifth away from any other note.
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What's all this about inversions? Well, the trick here is that scales "roll over" every octave. In other words, a note gets the same letter name, no matter what octave you play it in.
Here's an example. The third (lowest) string of my ukulele is a C note. But if I fret the third fret of the first string, that makes a C note also. The difference is that the C note on the third string is an octave lower than the C note I'm fretting on the first string. Or, looking at it the other way, the C note I'm fretting is an octave higher than the open third string.
So if a C major chord is C-E-G, we can take that first note and move it an octave higher. It's still a C note, right? Now our chord looks like this: E-G-C.
That's called "first inversion". The lowest note is now the third of the chord--the "E". The root is no longer at the bottom, but that's okay. It's still a C chord because we've got the right notes in there, regardless of what octave they happen to be in.
But why stop there? We can take that new lowest note and move that up an octave too. Like this: G-C-E. Now the fifth is the lowest note. That's called "second inversion".
You'd think we could do this one more time and get "third inversion". Nope! If we do try it, we get this: C-E-G. We're back to root position again.
(By the way, the open-position F chord on a standard uke is always in second position. That's because the lowest-pitched note is the C, third string, which is the fifth of an F chord. What inversion is the open-position G chord in? How about the A chord?)
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We've been dealing with the major chord formula so far, but there are plenty of other types of chords. A minor chord, for example, is made up of a root, a minor third, and a fifth. In this case the third is a half-step lower than it is in major chord. That's the same thing as saying that the third is one fret lower. You can also say that the third is "flatted". All three things mean the same thing.
This is all because the chord is based off of the minor scale, which (among other things) has a flatted third in place of the normal third. The fifth is the same in the major and minor scales, so we don't have to change that.
So a C minor chord is C-Eb-G. And an F minor chord is F-Ab-C. The third moves down (is flatted) one half-step (or one fret). Make sense?
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An augmented chord is not as common as the major and minor chords. That's what the + means (you might also see "aug" after the chord). The formula for it is almost the same as a regular old major chord, only the fifth is made sharp (or "augmented"). You can also say that it's raised by a half-step, or raised by one fret. So a C+ is C-E-G#.
The scale for Gb is this: Gb Ab Bb B Db Eb F
So a Gb major chord is Gb Bb Db.
And a Gb+ chord is Gb Bb D. (If you sharpen a note that's already flat, it "unflattens" it.)
Inversions work the same way for augmented chords as they do for major chords. Gb+, second inversion, is D Gb Bb. The augmented fifth is now the lowest note in the chord.
Whew!
JJ