How would I go about creating non-diatonic chord progressions. By diatonic I mean chords that come from a major or minor scale. (I ii iii IV V vi vii).

I want to be writing similar to Opeth, but I have no idea how they do it.
You mean to go atonal? If you want to then the best way would be to break a few rules. Atonal 21st century music is more focused on timbre as opposed to tonality, so if you don't like how it sounds, try changing the way you play the notes instead of changing what notes you play. Try working with any sound effects you can do with a guitar or whatever instrument is available, this would be a good time to include a keyboard.
Quote by pwrmax
You mean to go atonal?
Playing Dm Bb Bdim C does not stick strictly to the Dm scale, but it certainly atonal.

There's a nice non-diatonic progression for you, TS.
there are unlimited ways to go about this. if your looking for solutions through music theory the amount of scales you can use is inumerable.

melodic minor is very popular in metal, classical, and jazz. although it is for the most part minor it is not diatonic. it is minor with a sharped 7th tone. C melodic minor: C D Eb F G Ab B

a chromatic (twelve tone) scale, which basically means you can play any note/ chord you could conjure up, which can make for very strange but cool music.

a pentatonic (5 tone) scale used in a lot of music from rock to blues to jazz to world musics.

there are Arabian, Asian, Indian, Neapolitan, and a **** load of others.

another way of using different chords is by constructing a lead line to which the chords follow (rather than vice versa), be it guitar, vocals, kazoo, bass, whatever; you can use whatever chords you like assuming it follows the lead line. it is called modulation and it is the process of changing keys. many musicians do this in many different genres.
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^you're thinking of harmonic minor. melodic minor has a natural 6th, and a natural 7th.

also, isn't pentatonic just a variation of the major scale and therfore diatonic?
There are different types of chords. I usually list them as tonic chords (or chords being used as a tonic), passing chord, predominants and dominants.
Generally, most progressions will follow a pattern like this:

Tonic chord (i/I chords) - passing chord - predominant - dominant (or a substitute for a dominant chord) - tonic

The tonic chord is just a major/minor chord. You can't use augmented or diminished chords as tonic chords.

Any chord (from any key) can be used in the passing chord, just as long as it fits between the tonic and the next chord and has nice voice leading. You can skip this chord if you're not too good at voice leading.

You don't really need a predominant chord, but it's nice for setting up a dominant chord. There are millions of predominant chords, most of them are non-diatonic (which answers your question). Some common non-diatonic ones are augmented sixth chord, neapolitan chords and secondary dominants. Wiki these.

The dominant chord is usually a major or dominant seventh chord. You can substitute this chord for a diminished chord 4 semitones up (so Bdim can sub for Gmaj) and you can even use a tritone sub (which is non-diatonic). From these chords, you can either modulate to another tonic (possible if you're using a tritone sub) or go back to your original tonic and then use your old tonic chord as a dominant, predominant or passing chord in a new key.

So I could do this: Dm (predominant) - G7 (dominant) - Cm (tonic for G7, predominant for the next key) - F7 (new dominant) - Bbm (tonic for G7, predominant for the next key) - and so on.

So if you alot of modulation and alot of non-diatonic predominants or tritones subs, you can have have a 'non-diatonic' chord progression (which is different to atonalism btw)

This works for alot of jazz progressions. Alot of classical progressions are a bit more complicated. Classical progressions usually make use of deceptive cadences and imperfect cadences, both of these are usually used at the end of phrases that don't resolve (only the last phrase of a movement usually resolves).

BTW, I don't think opeth uses non-diatonic progressions. It's mostly jazz songs that have all those fruity progressions.
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Quote by mikeman
^you're thinking of harmonic minor. melodic minor has a natural 6th, and a natural 7th.

also, isn't pentatonic just a variation of the major scale and therfore diatonic?

You mean major 6th and major seventh. Saying natural can be a bit misleading and confuse someone.
Quote by mikeman
also, isn't pentatonic just a variation of the major scale and therfore diatonic?

Perhaps it's not so much that pentatonics aren't diatonic to a key (because they certainly can be), as much as using only pentatonics can make the music not function tonally in the Western sense.

Something like Pagodes by Debussy. He's using pentatonics, and while you might not say those sections are out of a key, there isn't really so much harmonic movement in the Western sense (with functions like demon described).
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