#1
I was wondering how you would use 7th Chords in a chord progression? Is there any theory to it, or is it personal preference?

Chord Progression:
Cm Gm A Fm

Would I just use Minor 7th Chords to replace the minor chords and Major 7th chords to replace the A chord?
#2
Quote by _Joey_
Would I just use Minor 7th Chords to replace the minor chords and Major 7th chords to replace the A chord?

Someone else can explain the theory better than me but pretty much. You could also use just a 7th chord - A7 - aka a dominant 7th chord on the A. That has the major third C#, and the minor seventh G.
#3
Wherever they sound good.

They have much more complex sound, so you wouldn't necessarily substitute any triad with a 7th chord. In fact sometimes the mood of the 7th chord is completely opposite of the related triad. Maj7 chords can sound wistful or melancholy, and min7 can sound upbeat.

Also the A chord in that sequence seems really out of place. Did you mean Ab?
#4
Quote by cdgraves
Also the A chord in that sequence seems really out of place. Did you mean Ab?
cdgraves
Yeah, I forgot to add the flat in there.
#5
It really depends on the mood you're trying to achieve. 7th chords in general have a really chill and relax kind of mood, so if you're going for a really relaxed floaty sound, then you should definitely use the 7th chords. I recently wrote a song that has a similar chord progression to this. Cm7 Gm7 Cm7 Fm7 Ab7, and I intentionally wrote it that way cause I wanted the harmonized leads behind it to sound really floaty and melancholy. 

With your chord progression I'd personally do Cm7 Gm7 Ab Fm. But that's what personally sounds better to me, and it also just feels better on my fretting hand, especially when I go from Gm7 to Ab.

Hope that helps!
#6
Generally, the minor seventh chord nearly always works well instead of a straight minor chord.   For major chords it's a bit trickier and much more context dependent, either the major 7th or the dominant 7th will work, depending on the role of the chord in the progression and the melody- you just have to use your ears for those. 

In that progression you could switch each of those chords to a 7th chord - so minor 7th for the minors and major 7th for the Ab ( you could also use the dominant 7th for the Ab, which works very well).  You could also switch the G minor to a G7 , which would also work given the progression. 
#7
Learn about chord functions and it may become more clear what kind of seventh you could use. But it's really all about using your ears. What sounds good is good.

Cm Gm Ab Fm sounds like the key of Cm to me so you could just look at the notes in the C minor scale. So if you wanted to stay diatonic to the key and add sevenths to all of the chords, your progression would become Cm7 Gm7 Abmaj7 Fm7.

Generally you can replace pretty much any minor chord with a m7 chord and it will sound just fine, though that may not be the sound you are after.

When it comes to major chords, it has a lot to do with the function of the chord. If the chord has a dominant function, using a dominant 7th chord makes sense. But if it doesn't have a dominant function, maj7 will most likely work better. Then again, some dominant 7th chords don't have a dominant function (for example if we are in the key of C and use an F7 chord). Just use your ear. As your ear gets familiar with the different chord types, you will know how to achieve the sound that you are after.

There aren't really that many possibilities: A minor chord is pretty much always a m7 (if it has a seventh), and a major chord can be either a maj7 or a (dominant) 7.
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#8
_Joey_ 

eveyone gave you good advice..but if you are serious about unlocking the chord mystery..do study diatonic harmony..it is not that difficult and will unlock how chords work together within a scale structure.

while just adding a major 7 to a triad gives the chord a much richer flavor..you can also extend the sound by adding the 9th 11 and 13th tone to the chord and then of course you can alter the chord with a flat 5 / sharp 11 and other such alterations maj 13#11  and so on 

same with minor chords the minor 7 can also have a mi7b5  minor 9  minor 11 and minor 6 and minor 6/9 

dominate chords have many ways to be extended and altered Dom 7 9 11 13  / 7b5 7b9  for single alterations and double alterrations 7b5b9  7#5#9 and others

as Marine said--learning how chords function in a progression gives clarity on how to use it...here is where the study of theory comes in..again it not that hard but requires some time and dedication

hope this helps
play well

wolf