Okay, go back to the C major scale, which you’ll remember is the white keys from C to C, and includes the notes C-D-E-F-G-A-B-C.
Your first triad, or three note chord, is the C major chord. It involves the first, third and fifth note of the C major scale- C-E-G. Play them at once, and you have a C major chord. In the c=scale f C it’s also called the Major First, and sometimes indicated with an upper case Roman numeral “ I “
Now, if you move each finger one note to the right in that scale, you’ll get a D minor chord. ( D-F-A). In the scale of C it’s also called the minor second (we’ll get to why later ) and sometimes indicated by a lower case roman numeral “ ii “.
By continuing up the scale moving each finger to the right by one key, you get seven chords based on the C Major Scale:
C-E-G = C Major= Major I
D-F-A = D minor= minor ii
E-G-B= E minor = minor iii
F-A-C= F Major = Major IV
G-B-D = G Major = Major V
A-C-E = A minor = minor vi
B-D-F= B diminished = minor vii diminished
The numbers represent the scale degrees within a scale, 1-7.
The Roman numeral is capitalized when the resulting chord is Major, and lower case when the resulting chord is minor.
It is possible that some of the same chords may exist in other scales, but they will be numbered differently based upon the key they are in. For instance, the same G Major Chord used in the C scale is used in the G major scale, but it’s the first chord in that key, so it would then be called a Major I.
If you are new to playing with chords on a piano, try playing each of the chords in a C major scale in succession, with each hand, and then with both hands together. Getting into chords is fun, it’s where the music starts to sound like music.
Also try playing each of the notes in a chord in succession, which is called and arpeggio. You can play the notes individually, or hold each note as you play the next for variations in sound. Try playing up and down the keyboard with variations on this theme. For instance, play each note of the C chord in your left hand holding each note as the next is played, and then in the right hand, making the six distinct notes come in at different times and end at the same time. Do the same thing for D-G-G, and so on. Try it, it sounds pretty! Come up with some of your own exercises to play around with involving the seven chords based in the key of C Major.
The notes for these chords are the same for a guitar, although I think the relation between the notes is visualized a little easier with the layout of a piano or keyboard. The basic theory however is the same, and can be applied to multiple instruments.
Instruments which can play more than one note at a time frequently make use of chords. Other instruments which sound one note at a time may use similar concepts such as the arpeggiated notes of a triad. They may also represent one note of a larger chord made by multiple instruments playing at once in a band, orchestra, or chorus.