In order to understand chords, we must first understand scales. Any note you play has corresponding scales. Two of these scales are the major scale and the natural minor scale (there are three types of minor scales, but this entry will only discuss the natural). Every major scale follows the same pattern and every minor scale follows the same pattern.
The pattern between notes for a major scale are:
Whole step, whole step, half step, whole step, whole step, whole step, half step.
Look at the C major scale for example: C, D, E, F, G, A, B, C. If you examine the intervals between those notes, you will notice they follow the major scale patten.
In case you’re wondering what whole steps and half steps are - a whole step refers to skipping a note to get to the next note, C to D skips over C#, and a half step refers to playing two notes directly next to each other, C to C# is a half step interval. In piano terms a whole step translates to skipping a black or white key between notes, and in string instrument terms this refers to skipping a fret between notes. A half step translates to playing two keys or two frets right next to each other.
For a natural minor scale, the pattern is slightly different:
Whole step, half step, whole step, whole step, half step, whole step, whole step.
Look at C natural minor: C, D, Eb, F, G, Ab, Bb, C. You'll find that the intervals match this pattern.
Now that you have a basic understanding of scales you are ready to construct chords. Surprisingly, major and minor chords are fairly similar. Every major and minor chord uses the first note, or the root of the scale, the third note of the scale, and the fifth note of the scale. For a C major chord this translates to: C, E, G. For a C minor this translates to: C, Eb, G. Major and minor chords share two of the exact same notes; the only difference is the third note of the scale is different by a half step. Pretty remarkable that the only difference between the happiest of chords and the saddest of chords can be a half step, but now you have the theory to prove it!
Now, here comes the theoretical extra credit. By knowing what key a piece of music is in, you can play seven different chords and still stay in the right key! If someone wants to play in the key of C major, you know that you can only play the notes: C, D, E, F, G, A, B. It's one thing to come up with fun melodies using these notes, but you can also construct a chord with each of these notes and still stay in key. For instance, on the second scale degree of this scale, our D chord would have to be minor (D, F, A). Playing a D major chord (D, F#, A) would sound off because F# isn’t in the key of C major. So what are the rules to follow then in terms of playing the right chords?
Major scale chord pattern:
Play a major chord on the first scale degree, a minor chord on the second scale degree, another minor chord with the third degree, a major with the fourth and fifth, a minor with the sixth, and a diminished with the seventh (A diminished chord means you play a minor and also bring the fifth of the chord down a half step. For a B diminished, this means you play the notes: B, D, F instead of playing the usual F#).
Minor scale chord pattern:
Play a minor chord on the first scale degree, a diminished chord with the second scale degree, a major with the third degree, a minor with the fourth and fifth degree, and a major chord with the sixth and seventh degree.
Learning how scales and chords are constructed will help you understand how musical sounds fit together. Additionally, a knowledge of music theory will make it easier for you to play with others. I know it can be a bit confusing at first, but try incorporating what you learned today into your next practice session, and I promise it will all begin to click!