Related Chords, the Key and Changing Keys.
Here's a HUGE lesson for those who have yet to delve into the theory side of music. It's certainly not necessary to know all of this stuff to play guitar. If your aim is to learn some well known tunes and be able to strum along, then you can simply learn the chord positions, commit them to memory and enjoy the sounds. One day, though, you'll start to wonder WHY certain chords work well together, why certain combinations seem to come up more often than others and how it all hinges together. If you've reached that stage, then hopefully this lesson will shed some light.
I've used the simplest song in the world, Happy Birthday. The main thing this lesson teaches is the way chords relate to each other within a KEY. I often refer to the Roman numeral value of chords and I know it's a confusing topic when you first hear about it. Hopefully this will help you understand. What I'm playing here is the chord progression to Happy Birthday in several keys, in other words I'm CHANGING keys each time I get to the end. It starts off in the key of C, then modulates (a fancy way of saying 'changes key') to D, then E, Then G, then A, then back to the original key C.
The Playing aspect
Before I get into the theory aspect of it all, I'll get the actual physical playing part out of the way: the right hand is using a very simple plucking pattern throughout: it's in 3/4 time (three beats per measure) and the thumb plucks the bass note on the first beat of each measure, then the three fingers as a unit pluck the remaining three notes in the chords I'm using. I'm only using 4 note chords. The bass note each time is the root of the chord. It's not an award winning plucking pattern -- I'm just trying to keep it as simple as possible -- but it's still a good discipline to play along and get it as clean and accurate as you can.
The left hand is just playing the familiar open chord shapes, nothing too demanding there. You'll notice that I didn't actually play a B chord at one stage, I played B7. I was just avoiding having to use a barre chord.
The Theory aspect
This is where it's best just to watch and, most importantly, to LISTEN.
I've indicated in the movie the Roman numerals for each chord. You'll see 'I', 'IV' and 'V' next to the chord names. In plain English, you'd refer to them simply as "The One Four Five Chords". Those numerals are indicating the 'rank' within the key that each chord has. This tune uses the most commonly used chords from the key, the three major chords and they form the backbone of just about every tune out there. (There are four other chords in that emerge naturally in a key, they are the ii, iii, vi and vii chords. They are minor chords and the lower case Roman numerals reflect that. The vii chord is minor/flat5 ... no need to get into the details here.)
The progression for Happy Birthday is:
|I - - | V - - | V - - | I - - | I - - | IV - - | I - V -| I - - | There's that one bit near the end where a V chord appears for just one beat.
Notice that in the movie I made them green, except for that orange V that keeps popping up. That orange V should also sound quite different also, it should make you ears sit up and take notice, telling you that we've moved out of one zone and now we're in another. The orange V chord is the V chord from the new key, and we've made it a 7th (which is its natural extension anyway) chord to make it even more effective. The important point to note is that the new zone is exactly like the last, but on a new level, a different pitch, and that V7 chord is a sort of transition chord, priming your ears for that new level. The proof is in the Roman numerals ... if you follow them in the new key, you'll see that they're the same each time, and you can keep singing Happy Birthday each time. Each key has the same rules, the same relative sounds, the same everything.
Imagine a sporting complex with 12 tennis courts labelled A, Bb, B, C, C#, D, D#, E, F, F#, G and G#. Some are a little slipperier than others, some have lighting problems and swirling wind problems, so you do have your favorites, but they're all identical in size, layout and the rules of tennis apply to all, as does the scoring system. You're always going to be playing tennis no matter which one you wind up on. Keys are a bit like that. Look at the Music Building Lesson too, for another analogy.
This example is about as basic as it gets, using just the three major chords in a very straight forward way. I've only used the easy guitar keys here -- C, D, E, G and A -- but of course there are 7 other keys, and all keys also have three minor chords that relate in the same way.
Here's another interesting fact: You'll notice that certain chords come up in different keys, BUT NEVER AS THE SAME ROMAN NUMERAL. For example, there's a G chord that pops up in three keys: once as the I, once as the IV and again as a V. However ... I don't see it as the same chord. The notes are the same, but the context of each of those G chords is so different that I just can't see them all as just the same 'G chord'. That's how ingrained this way of perceiving and tracking music has become for me after all these years.
The D chord also appears in three keys, so does the A. If I had done all 12 keys, all major chords would appear in three keys, once as a I, once as a IV and once as a V.
I've often mentioned the 'function' of the chord ... I think the power of that V7 chord is the best example. Here, you can see and hear how a seventh chord, popping up out of nowhere, instantly let's you know that we've entered a new key, one where it's tagged the V chord.
Below is the tab and I've worked on it a bit to help show this in graphic format. The vertical columns show how each key follows the same Roman numeral pattern. The orange column to the left shows that V7 chord, the transition chord, the one that tells our ears that something new is happening. You can see that the last orange chord is G7, which leads us back into the original key of C.
I've included a midi file of the whole thing with the Happy Birthday melody so you can hear it moving up and down pitch too.