Whether you want to write songs or are trying to figure out how to play songs that you would like to play, it is important and helpful to understand the natural order of chords for progressions.
There is a formula that tells you exactly where and when minor chords are called for and when to use major or dominant seventh or diminished chords within a key.
*Here is more info on building chords and scales if you need this background info first.
Now, we all know that rules are made to be broken (sometimes) and so it is not necessary to always abide by this formula. You can't begin to break the rules, however, until you know what they are.
Which Chords Belong In Each Key?
Major Keys:For any major key, the chords available are:
I. The Tonic- in the key of C, this would be a C major chord.
ii. The next chord is built from the second note of the key scale, it is a minor chord (ex: in key of C, ii. is D minor).
iii. The chord derived from the third note of the scale is also a minor chord.
IV. The chord built on the fourth note of the scale is a Major Chord.
V. The chord built on the fifth note of the scale is the dominant chord. So in the key of C, this would be G7.
vi. The chord built from the sixth of the scale is a minor chord (the relative minor to the key chord).
vii. The seventh chord in the series is the diminished chord.
Again, these are the chords available for a progression in any major key:
Upper case Roman Numerals denote Major chords, while lower case numerals indicate minor chords.
Often the seventh chord is flattened and played as a major chord instead, as B flat in the key of C.
Minor KeysIn a natural minor key, the chords are:
But, as you know from our discussion on scales, there is more than one minor scale. In fact, the above is not often used! Instead, for a minor key, you can also build chords using the harmonic minor scales, in which case you would raise the seventh chord a semitone and include G# rather than G. You would also include chord V as a Major -or Dominant seventh- chord when using the harmonic minor scale for chord progressions.
Now the above are the rules; these chords are the only chords that strictly belong in any key.
You Can Break The Rules
These rules are broken all the time, by many artists in many songs. These are just guidelines.
How To Break the Rules:It is common to change a major chord to a minor chord instead. This seems to happen most often on the IV chord, being changed to a minor, in a song in a major key, or the V chord being changed to major in a song in a minor key (which is within the rules anyway when using harmonic minor scales).
You could change any of the major chords you wish to minor if you think it works for your song.
Or vice versa; what is supposed to be a minor (the ii, iii, or vi, in a major key) is sometimes played as a major chord instead. This happens all the time in popular music.
Another popular way to break these rules is to throw in a flattened chord, particularly a flattened seventh, rather than the diminished seventh. The fifth and or the third are flattened in many songs as well. Less frequently, the sixth has been flattened as well.
You could include a chromatic, semitone shift anywhere in your progression, using both the called for chord as well as flattening it-- rock music has made the semitone shift a characteristic sound since its beginning.
You could also use dominant chords in a position other than with the called for V chord in a major key or III chord in a minor key.
Not to mention using major7 chords or subbing other chord variations (such as sus and extended chords) wherever you'd like. I like to extend into extra bars by varying the chord different ways- from the I to the Imaj7, or with a sus, back to the key chord, then the other sus and back to the key chord again, for example.
The simple song I was working on today moves from a C chord (as the IV) to a Cadd9 before moving to D before D7, for example.
You could imply the chords rather than playing them outright. You could do this with treble notes against a bass line, or leave out the treble altogether and imply the chords using just a bass line instead. This might be just the thing for your bridge or another song section.
Also, remember that you do not have to stay in one key! Key changes, called modulations, do not break the rules, but knowing the rules for which chords fit in which key will help you to make successful and easy key changes when you recognize the available pivot chords that are common to both keys.
Working in a key change may be the most interesting thing you can do to enliven a boring song.
Source:I want to tell you that I was introduced to and learned much of this information by the best and most helpful book I've ever read on the subject of music: How To Write Songs On Guitar, by Rikky Rooksby. This book is extraordinarily worthwhile if you want to know how songs are put together, especially regarding chord progressions and song structure.
I read this book as an absolute beginner musician, with fumbling fingers on my first guitar, and it was the most helpful of any of the many music theory books I had yet read in my quest to understand music. I learned a lot and, as a result, was able to entertain and impress myself with writing my own songs from this beginning, rather than exclusively frustrating myself trying to play songs which exceeded my skills!
I checked this book out again a couple years later, when my skills were much better established, and learned still more on second reading. In fact, I renewed this book twice from the library and took copious notes! You'll learn a lot from this book too, much more than the basics included in my short lesson. This book and Ricky Rooksby's other excellent books are worthy books for adding to any musical library.
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I hope you have some fun making music today!