Before we get into some of the basic chord progressions in modern music, let's discuss some points on notation. The Nashville Numbering System is a method of denoting chord letter symbols by using roman numerals. For example let’s say we take one of the more common progressions known as the I-IV-V, or the 1-4-5. If our band mate says this song is I-IV-V in the key of G, than that means the chords are G-C-D, if it is the key of C, than the chords are C-F-G. Here is a helpful pdf reference sheet for you to look at during this explanation.
The I chord is the tonic chord, the root chord that all other chords are based off of, for now on we will use the key of C, so C will be our tonic chord. The ii chord is the supertonic chord, which is one musical step above the tonic. The reason it is lowercase ii and not II is because the supertonic is often a minor chord. So in the key of C the ii chord is Dm. The mediant chord or the iii chord is also a minor and is the least used in most chord progressions. For the key of C the iii chord is Em and is a very weak chord compared to the others in the key. The IV chord is an F major which is known as the subdominant, it is very commonly used in many progressions and is one note below the dominant. The V chord is the dominant chord, in this case a G. This chord is the second most important to the tonic or root chord of the progression, the dominant is used for tension as it often wants to resolve to the back to the tonic. And then we have the vi chord, another minor chord and in this case Am. This is known as the submediant chord and provides a great contrast with the tonic. And finally we have the vii chord, the leading tone. This chord is played in a variety of ways but always has the feel of needing to lead right back into the tonic, thus a leading tone.
Chord progressions are all built around these particular tones and chords in each key. The question might be, does it matter what key we play in? Well the answer is, it depends. Even though a I-IV-V song can be played in the key of C as C-F-G, or in the key of G as G-C-D, each key can give the song a slightly different feel. Sometimes musicians use the Nashville Number System to quickly transpose keys, to make it easier to play a song, but they occasionally find that the “feel” of the song can change with the key. Each chord progression out there is based upon the relationship between these chords. For example listen closely to how each of these progressions sound; C-G-Am-F and C-Am-F-G. The first one is one of the most common pop rock progressions, and the second one should sound familiar because it is the basis of nearly every Doo Wop/50’s song ever. The first progression is the same as Let It Be and Don't Stop Believing, while the second progression is the same as the songs Earth Angel and Stay.
When you learn a new chord progression it is important to pay attention to the order of the notes and how each chord leads into another sound or resolves. During a middle of a verse we want chords that lead us on; during the end of a verse we want chords that have a resolution. Usually a song will begin on the tonic and end on the tonic, of course there are exceptions. And we can change a common progression by giving the chord a slight change, by adding a 7th for example. Go back to that common I-IV-V progression, now let’s add some sevenths and see how it is changed. Instead of E-A-B let's play E-A7-B7... notice how it sounds far more bluesy and rocking? In fact many music historians will tell you that the 7th is what makes rock... well rock! Even the Beatles tell a story of riding a bus across town just to learn how to play a B7 chord (remember they had no internet back then!) which allowed them to play like their American rock heroes.
Of course that is how new music is made, if we all play the same I-IV-V progression it will get a little boring, so we make substitutions, we add 7ths, 9ths, 13ths, we change majors to minors, minors to majors... the key is to make sure the music still resolves to an acceptable place. If we change C-G-Am-F to C-G-A-F, play that and notice how something seems off, it just doesn't have a pleasant sound. Of course maybe there is a musical time when this “off” sound might be great, but generally we want a progression that will make our ears happy!
So now that we have an understanding of chord progression basics and the Nashville numbering system, let’s look at some of the more common and simple progressions out there. We will stay in the key of C for all examples. One of the most basic progressions used in folk music and reggae is I-V, or C-G. These simple two chord songs can be very popular such as Camptown Races or Alouette and even the Beatles Yellow Submarine. Often a 7th is added to the V or dominant chord in these songs. Another popular progression is the blues progression I-IV-I-V-I and of course 7ths can be added to make C-F-C-G7-C. There are so many variations on the blues progression and it doesn't take much skill to play around with it, early blues songs and early rock are based around blues progressions. In Rock n Roll another common progression is I-bVII-IV, now that means the seventh chord is a major and flat. C-Bb-F is used by Roy Orbison in You Got It and Boston in their song More Than A Feeling, in fact variations of this progression are well known by those in rock and pop rock. And finally another basic one is the pop rock lydian progression of I-II-IV-I, notice that the II is not a minor (ii) in this situation. C-D-F-C was used in Eight Days A Week by the Beatles and Mr. Tambourine Man by the Byrds.
Now sometimes these chord progressions mentioned above are used for verses, choruses, and sometimes bridges. Rarely is the same progression used throughout the entire song; however it is important to recognize these progressions. After a while you will start to pick them out when you hear new songs, and even later still you will learn to appreciate subtle changes that songwriters make to common progressions. That way they can create a song that will be liked, yet will still be a little different than the rest. Remember to always pay attention to the way in which various chords sound with each other, a tonic, dominant, and so on mean nothing if they cannot work off each other. Each time you now learn a new chord, notice how it sounds when played before or after other chords you already know. Experiment as much as possible with chords, keys, and as many chord progressions as you can find!