Hmm. Diatonic Chords. I bet you're wondering how those are different from regular chords. Well, actually they ARE just regular chords. "Diatonic" means that they all refer to a particular scale. These chords are ones that will all fit in a diatonic scale because they are made from it.
To form the diatonic chords for a certain key, we will start by writing out that scale. Let's use A:
A B C# D E F# G# A
Obviously the A chord is going to be the 1,3,&5, so A C# E. (If you are lost, you might want to review the major triads lesson .) We're going to call this the "I" chord because it is based on the first note of the scale.
Now if we start on the D note in that scale and do the same thing: 1,3,5 = D F# A: we get a D chord. It is based on the fourth note of that scale, so we'll call it the "IV" chord.
Now the same thing goes for the E note: 1,3,5 = E G# B: we get an E chord. Since this is based on the fifth note in this scale, we'll call it the "V" chord.
Notice that all of these Roman numerals are capitals. That is because they are major chords.
Let's take a look at the B note and do the same thing: 1,3,5 = B D F#: THIS is a minor chord. It starts on the 2nd, but since it is minor, it will be lower case, so we'll call it "ii".
Same with C#: 1,3,5 = C# E G#. This starts on the third, but is minor, so "iii".
Now for the sixth note: 1,3,5 = F# A C#. Again, minor. So: "vi"
Now last but not least is the seventh: 1,3,5 = G# B D. This chord has a minor third in it (from G# to B) so it is lowercase, but it has a diminished fifth in it (from G# to D) so we put "dim" with it as well so that it looks like this "vii dim".
So here is a list of the types for the diatonic chords in any key:
R = I (major chord starting on 1)2 = ii (minor chord starting on 2)3 = iii (minor chord starting on 3)4 = IV (major chord starting on 4)5 = V (major chord starting on 5)6 = vi (minor chord starting on 6)7 = vii dim (diminished chord starting on 7)
So in the key of A:
and in C:
Hope this helps! Practice hard and let me know if you have any questions!