There seems to be some interest in learning about chord progressions here, so I figure I will give my take on the subject. All of of what I have to say is purely my own impressions. I've been interested in chords from early on. And I've found making a conscientious study of them makes music more understandable and repeatable. I will also go at the discussion by including several styles of music, instead of concentrating on just bluegrass. But since Bluegrass is what a number of people seem to play on the list, I will always have ample examples from that.
I should begin by explaining how the major scale is created. The first note of the major scale is the name of the scale. By that I mean a "C" scale starts on a "C" note. An "A" scale starts on an "A" note, and so on. There are twelve different scales if you don't include double flat or double sharp scales. Are you with me so far?
The twelve major scales are:
C, C#, D, D#, E, F, F#, G, G#, A, A#, B
The C# D# F# G# A# notes also have "enharmonic" notes related to them. By that I mean the C# is played in the same place in most instruments as a Db note. They are the same pitch; they just have different names. The tricky part is that a C# scale has a zillion sharps in its key signature, and the Db scale has a bunch of flats - plus the C# scale starts on some kind of a "C", where the Db scale starts on some kind of a "D". So even though the scales are played on the same place on the fingerboard, the notes have different names depending on whether you are playing C# or Db. I'll show you why in a minute.
The "C" scale is the simplest one of all, because there are no sharps or flats in the scale:
C D E F G A B C
These are the white keys on the piano in order. The distance between each note of the scale on the keyboard or fingerboard determines the structure of the major scale. In the C scale, "C", as I said before is the first note of the scale. "D" the second note of the scale is two steps (frets) higher than the "C" note. The distance between two notes is called an interval, and a two-fret distance is a whole step interval. The next note of the scale is "E" which is a whole step away from the "D". Have I lost anyone yet?
The kicker is the next note -- "F" -- is only one fret away from the "E". This interval is called a half step. There are other names for the intervals as well. A whole step interval is called a major second, and the half step interval is a diminished second. But I would prefer to avoid that whole discussion as it just clouds the issue for now. Continuing on, the next note is "G", which is, once again a whole step from "F". "A" is a whole step above "G", and "B" is a whole step from "A". Finally, we close off the scale with another "C" note, this one is a full 12 steps higher in pitch than the first "C".
So, a major scale in western music is made up of a sequence of whole steps and half steps in the following order:
Whole Step, Whole Step, Half Step, Whole Step, Whole Step, Whole Step, Half Step.
To create the "G" scale, a very popular one in Bluegrass, start with a "G". Up a whole step is "A", up another whole step is "B". Then the note under the half step is "C". Up a whole step to "D", Up a whole step to "E". To go up another whole step we need to play some kind of an "F". We can't play the "F" note because it is only a half step away from the "E" and we need a whole step jump from the "E". So we have to play "F#". Because we have to sharp all of the "Fs" to be able to play in the key of "G", we annotate our sheet music with a sharp symbol where the "F" note fall on the clef. That save the person writing the music from having to put all those sharps in the music every time you play an "F". If you are reading music, you are supposed to automatically know to sharp all the "Fs". Fortunately we won't be reading music, so all of this theory is just for background understanding. We want to live in the "real world", and just play, and not get bogged down in theory. I would recommend you get a notebook of some kind and write out all the different scales in it, so you can find out what notes are in certain scales when you need to have that information.
For our purposes we are going to number the notes of the scale because I have found that chord progressions have a specific sound. And once you can recognize the sound you will be able to find ways of playing the chords you hear without having to go through a lot of agonizing theory. Here's my plan:
The first note of the scale is numbered 1, or as I prefer the Roman numeral I. The easy part is the second note of the scale is numbered 2 or roman numeral ii (I'll explain someday soon why I used the lower case, or small letters, for 2). Note # 3 is 3 or iii, 4 is IV, 5 is V, 6 is vi, 7 is vii, and 8 I don't bother with. So I'm just asking you to recognize 7 notes. For the "C" scale the numbers are:
C D E F G A B C 1 2 3 4 5 6 7 8 or I ii iii IV V vi vii I in roman numerals * * *
(notes in a "C" major chord)
To create a "C" chord we take the first, third and fifth notes of the scale ("C" "E" "G") to create the C major chord. Since there are three notes in the chord, this chord is called a triad. If we were to stack them up in order from lowest to highest notes (an impossibility on the mandolin due to its tuning) we would have a chord to play. If we rearrange the notes in the chord stack to, say, "G" "E" "C", we have what is called an inversion. That's why on guitar, mandolin, and other stringed instruments there are several different ways (shapes) to play a C major chord. We won't be concerned with much more about inversions until later. Suffice it to say it always good to learn several different spots on the instrument where you can play a given chord.
If we take the C scale and start it on the second note of the scale, "D", but not altering any of the notes in the scale, we have what is called the Dorian mode. It's a completely different discussion to go into modes here, we will touch on some modes later, but we don't want to complicate things for now. The Dorian mode looks like:
D E F G A B C D 1 2 3 4 5 6 7 8 * * *
(notes in a "D" minor chord)
Taking the first, third, and fifth notes of the Dorian mode gives us the D minor chord. To distinguish minor and major chords in the Roman numeral numbering system, I have the Major chords in capital letters, and the minor chords in small letters. In the Arabic number system, I will put the letter "m" to indicate a minor chord, i.e. D minor would be either ii (I may for clarity even call the chord ii minor), or 2m.
The next mode starts on "E":
E F G A B C D E 1 2 3 4 5 6 7 8 * * *
(notes in an "E" minor chord, or iii or 3m chord)
On "F" the mode is:
F G A B C D E F 1 2 3 4 5 6 7 8 * * *
(notes in a "F" major, or 4 or IV chord)
On "G" the mode is:
G A B C D E F G 1 2 3 4 5 6 7 8 * * *
(notes in a "G" major, or 5 or V chord)
Many times you will hear the 5, or V chord called a 7th chord, i.e. G7. Let's look at the "G" mode again:
G A B C D E F G 1 2 3 4 5 6 7 8 * * * *
(adding the 7th note of the mode makes it a G7 chord)
This is called an "extension." When you get into jazz and pop music, you play more harmonically advanced chords by adding extra notes to the chord stack to give it texture. Other extensions with the "G" chord could be:
G A B C D E F G A B C D E 1 2 3 4 5 6 7 8 9 10 11 12 13 * * * * * * *
9th Chord 11th chord 13th chord
You could even get fancier and sharp or flat the 7th and 9th notes to get a really way out sound. But this is not jazz theory, at least for now, so we will move on.
The "A" mode:
A B C D E F G A 1 2 3 4 5 6 7 8 * * *
(notes in an "A" minor, or 6m or vi chord)
The last mode starts on "B":
B C D E F G G A 1 2 3 4 5 6 7 8 * * *
(notes in a "B" minor 7b5 or B half dim., or 7m7b5 or viim7b5 chord)
Nobody seems to quite know what to do with the "B" chord formed here. To have a true minor sound, the "F" would have to be an "F#". What most theorists do is add a 7th note to the chord and call it a Bmin7b5. Others will call this chord a half-diminished. We hardly ever use the thing.
To recap, here are the chords based on the scale degrees of the key of C. You should figure out the other keys when you get the time.
C Major or I or 1
D minor or ii (iim) or 2m
E minor or iii (iiim) or 3m
F Major or IV or 4
G Major or V or 5 - Can also be V7 or 57 (this chord name is exactly why I prefer to name the chord using the Roman numeral system)
A minor or vi (vim) or 6m
B min7b5 or viim7b5 or 7min7b5 or B half dim.