Once you have learned several guitar chords and feel comfortable changing from one to another, you’ll probably notice certain patterns begin to sound familiar. for example, if you’ve been practicing transitioning from a G Major chord to a D Major chord you are playing a very common pattern. Many songs are written in the key of G and will use the D as a prominent chord in the song. The first two chords in “Freebird” and “Knockin’ on Heaven’s Door” (as well as countless others) are G and D just to name a couple.
Now let’s pretend that you happen to figure out the first chord of a particular song. Most of the time that first chord will be the key. Once you know the key, you’ll know the most likely places to look for the other chords in the song. So for this example we’ll start with a G Major chord. That means that the chords you’ll probably find in the song are these below:
I say probably because there are no guarantees in this process but the chords above are the most likely to be found. As you can see, the “I”, “IV” and “V” chords are all Major, while the “ii”, “iii”, and “vi” are minor. Most people can distinguish the sound of a Major (usually described as bright or happy) from a minor chord. (sad or gloomy) Because of this we can narrow down our guesses. If the song starts with a G chord and then goes to a “sad” chord it is probably an “am”, bm, or “em”. If the second chord sounds “happy” it is most likely a “CM” or a “DM”. There are literally thousands of songs that will follow the patterns in the diagram, but some won’t as I mentioned above.
Here are a few exceptions you’ll run into at some point:
Instead of the “viiº”, a “bVII” is often used in rock music. Notice the “flat” symbol in front of the “VII”. The large Roman numerals tell us that the quality of the chord has been changed to Major and the “b” symbol lets us know that the chord is built on the note that is 1/2 step lower than usual. In “Freebird” that is exactly what happens when the FM chord is played. Technically, the FM chord does not belong to the key of G Major, but it has been used so frequently that it can almost be considered standard in many genres. If a song is in the key of C Major and a “bVII” is used, it means we are talking about a “BbM” chord.
In gospel, R&B, and blues, the “iii” is sometimes replaced by a “III” chord. “Georgia On My Mind” is an example. This song is in the key of G major and uses a “BM” instead of a “bm”.
A common technique in traditional country music is the use of a “V7” chord from the neighboring key in order to facilitate a brief key change or a “secondary dominant”. In “Help Me Make It Through The Night” the song is in the key of Bb Major, but uses a “Bb7” during the bridge as a transition to an “EbM”. This section causes the “EbM” chord to briefly feel like the key.