Chord Theory

This is a little different than the "Music Theory" page, maybe some repeat, but it's information I wish I had known growing up.  Specifically, WHY does music sound good?

Basically, "music" is just sound waves organized by math.   A note that you hear when plucked on a guitar or plinked on a piano is just a string vibrating a certain number of times per second.  The tighter the string, the faster it vibrates.  The "A" note, for example, is defined as vibrating at 440 times per second (also called "frequency" or Hertz, Hz).   It used to vary, between 432 and 455 until it standardized in the early 1900's.     Think of the string "whipping" a sound like the crack of a whip to your ear every time it crosses the center line.   A string vibrating at 880 times per second "whips" your ear exactly twice as many times as a string vibrating at 440 times per second., and so sounds very harmonious.  And so, a string vibrating at 220Hz also harmonizes with the 440Hz and 880Hz strings.  220 is a "low A" (your 5th string on a guitar),  440Hz is a "mid A" (2nd fret, G string) and 880Hz is a "high A" on the first string, 5th fret.   Mathematically, they're just multiples of each other.

In the simplest terms and round numbers, if you vibrate a string 1.5 times faster than 440, you get 660 Hz.    660 Hz is the "E" note.  The E note "whips" your ear and syncs up with the  A vibration every other time, also creating a harmonious sound.     If you take 440 and multiply by 1 1/3, you get 587 vibrations per second, which is the "D" note, which also syncs up and is harmonious with the A.   So, the old math guys and the old music guys decided to break up the sound wave spectrum into equal parts, where 440Hz, 587Hz, 660Hz and 880Hz all fit nicely.  And they used math to come up with other intervals that fit within, and sounded harmonious with the other vibes.  What they came up with is the 12 basic notes we use in music today.     Then they named them, starting with A, ending with G  (It's actually a wee bit more complicated than this, but good enough for us).

"A4" is how they label the A note at 440 Hz.  The A at 220 Hz is called A3,  the A at 110 is called A2, etc.

So they had 12 distinct tones or notes, which they could repeat up and down the keyboard, but they all didn't harmonize (sound good) with each other.   Playing an A note and an A# note at the same time creates an awful sound (called "dissonance").  They found that only 6 of the 12 notes sounded "good" with the A note (B,C,D,E,F & G), so they grouped them together and called it a SCALE.  In this case, it happens to be the A MINOR scale.  Early on most organized music was church music, and church music was mostly in the minor "flavor."

Playing in A Minor, using those notes (A, B, C, D, E, F, G, A) and resolving (finishing on the A note) gives a certain feel or mood to music.  You all know a Minor song when you hear it.  But if you start and end on the C note, playing the EXACT SAME NOTES  (C, D, E, F, G, A, B, C) you get a different mood or feeling.  So the C MAJOR scale is made up of the exact same notes as the A MINOR scale, just played and resolved differently.    Am = A B C D E F G A    C maj = C D E F G A B C

So, now that we have these notes defined, how do we put them together to form nice sounding chords?   To start with they just grabbed every other one.     From the C MAJOR scale they first took  C, E & G   ( C  D  E  F  G  A  B  C) and strummed them together.  Mmmm.  Sounds nice.  Let's call it the C Major Chord or "C chord" for short.

Then they tried D, F and A. ( C  D  E  F  G  A  B  C)  Good sound.  Those three notes make a "D Minor" chord

E, G & B. also good.   ( C  D  E  F  G  A  B  C)  These three make an "E Minor" chord.

F, A & C, really good.  ( C  D  E  F  G  A  B  C)  Let's call this one the F Major or "F chord."

G, B & D, really, really good.  ( C  D  E  F  G  A  B  C  D)  Well of course.  The G note vibrates exactly one and a half times as fast as the C note, so they "sync" up quite often, and make a very harmonious sound.  This is called the G Major or "G chord."

A, C & E sound really good too.  ( C  D  E  F  G  A  B  C  D  E )  Oh wait,  A, C & E make up the A Minor chord and shares two notes with the C Major chord (C & E).  They must be related somehow!

B, D & F are the last every-other grouping in the C Major scale.  Ew...  that makes the B -diminished chord.  Better save that for the jazz players....

So, for a song in the key of C Major, the chords that work best are C, Dm, Em, F, G, Am and Bdim.

We can do this for all the major keys, just by shifting the notes up one or two frets BUT KEEPING THE SAME SPACING between them.  It's all about keeping the ratios of the vibrations together.  Just because we start in a different note, the ratios of the vibrations are the same.  This is why capos work.

In the Key of G, using the same math ratios and our 12 tones, we get these seven chords:   G, Am, Bm, C, D, Em, F#dim.

But sometimes we play a song in the key of G, but there are chords that aren't listed above.   They are A and F.   They are sometimes called "borrowed" chords, as they are borrowed momentarily from another key to create tension.  Remember that the two other chords in G that harmonize the best are the C and D chords.  If we look at the chords in the C and D major scales we have:

C ->  C  Dm  Em  F  G  Am Bdim         The three most important chords in C are C, F & G.  The G chord is already in the key of G, as is C, so we sometimes "borrow" the F chord as well

D -> D Em F#m   G  A  Bm C#dim   The three most important chords in D are D, G & A.  The D and the G chord are also in the key of G, and we also borrow the A chord from time to time.

If you're playing a song in the key of G, and you go to the F chord (tension), 99% of the time you'll go to the C chord next to resolve that tension.

If you're playing a song in the key of G, and you go to the A chord (tension), 99% of the time you'll go to the D chord next to resolve that tension.

People often refer to the One-Four-Five or I IV  V .. this simply means that you're talking about the first chord in a key, the fourth chord and the fifth chord.   In G that would be G C & D.    Since the D note vibrates 1 1/2 times faster than the G, they sync more often than any of the other notes.   The mathematical relation between the first note of a scale and the fifth note is intense.  So intense they call it "perfect."  Hence, they talk about a "perfect fifth."    The fifth chord in a scale is so perfect that it's sometimes referred to as "dominant."  If it's played as a 7th chord  (D7 in the key of G) they call it the "dominant 7th."

The sixth chord in a key is minor (for example, Am in the key of C, Em in the key of G) and is referred to the Relative Minor since both keys share the exact same notes.   The notes in C Major scale and A Minor scale are the same notes.  The notes in G Major scale and Em scale are the exact same notes.   If you can sing a song in C, you can also probably sing a song in Am.

The Dominant 7th chord in the key of Em is the B7 chord.   Since the key of Em and G share the same notes, putting a B7 chord in a G song sometimes works.  You see that in the chorus  of Tennessee Waltz.

So, TL;DR  here are the chords you need to play in the following keys:

And the highlighted green  is why we guitar players basically stick to C and G and use a capo.... these are the easy to make chords!!!!