it's going to depend on the chords that are being used underneath.
For example, if you are playing in some type of A minor key and you run into an E Dominant 7th chord (which doesn't exist in the fully natural minor version of the scale, since you need a G# (the leading tone) to make an E Dom 7th chord), then you can't use the natural minor form of the scale (since you would have a G natural against a G# note....well, you could if you wanted that sound, but you might not really like that sound in that way).

The natural minor (the first one you listed) probably works better if you are playing more modally, thinking of it as perhaps A Aeolian.
Here is a quick example of the sound difference in chords between the natural and harmonic minors.

Play a one-chord five-chord and one-chord two ways:

Amin Emin Amin ( i - v - i )
this is for the natural minor scale

now play this:

Amin E7 Amin (i - V7 - i )
this is for the harmonic minor scale

you can go more deeply into it than this, but this is a quick example to perhaps clear things up a touch

On the other hand, if you are just playing over a static Aminor chord groove, and there are no other chords going on, you can use either one, since you wouldn't run into any clashing notes (namely the G# vs G natural)

 

to add to this, the G# is something called a leading tone, also known as scale step 7.
Scale step 7 likes to "resolve" to scale step 1 much of the time (depending on what era of music you are basing your theory on)
So in this case, when you play Amin E7 Amin, it's that scale step 7 (a.k.a. the "leading tone") that is mostly responsible for the E7 sounding like it wants to go back to Amin.
It gives it a kind of "finished" or "settled" sound when you finish on the Amin chord.
So you are hearing a definite A G# A pattern with the notes inside those chords.
If you look at it in natural minor instead, you could do something similar:
Amin Emin Amin, where you have the notes A G A in the middle.
G natural is not a leading tone, a leading tone is always a half-step below.
It still sounds like it makes musical sense to play the A G A notes, but it doesn't sound quite as strong as the A G# A does. It's also a little bit less "jarring" to some peoples' ears. It all depends on whether that's the sound you like, or if you like it in the piece you're playing.
Hope that makes a little bit of sense

The other rule about harmony and chords is that there are no rules, and the rules can change every 50 - 100 years or so as tastes change.
So, potentially you could play harmonic minor over the Amin Emin Amin chords and perhaps you could find a way to make it sound sensible to the ears. But lots of times when people do something like that they are using a different kind of harmonic theory, and it's not quite what it looks like.
But if it sounds good, it is good.

 

So what you're saying is I have to know the chords underneath that are being played, and just because something says it's in "A Minor" does not mean that it necessarily uses a specific set of chords?

And I suppose there's no way to know what they are ahead of time (unless they happen to tell you) so you just have to listen for them and be able to identify "ok, that's a dom 7th chord which I recognize because I know what it sounds like, so therefore I can use harmonic minor" or something like that?

 

Yes. You have to know the function of a chord before you slap any scale on. This is the reason:

Scales that work with Amin
A Natural Minor
A Melodic Minor
A Harmonic Minor
A Phrygian Mode
A Dorian Mode
A Half-Whole Diminished/Octatonic
C Half-Whole Diminished/Octatonic
Eb/D# Half-Whole Diminished/Octatonic
Gb/F# Half-Whole Diminished/Octatonic
2nd mode of G Melodic Minor
A Pentatonic
A Insen Pentatonic
E Pentatonic
D Pentatonic
just to name a few.

So what you need to be able to do, is be able to recognize which Amin is specified. Whether it is a VIm which wants you to use a Natural Minor, or a IIm which wants a dorian. So how can you know what kind of Minor chord it is? General rule is: if it's before a V7 chord, it's usually a IIm chord. After a V7, it's very likely to be a VIm substituting for a I chord. However, you may sometimes see something like Am7 - D7- Gmin7 - C7. Both Minor chords are treated as IIm chords.

Also: Chords you can use in the key of A minor:

Amin, Bmin7b5, Cmaj, Dmin, Emin, Fmaj, G7, Cmin, D7, E7..... and on and on and on.

 

I have not read the other replies. So this might have been said before.

Theory can tell you when you can use different scales, but nothing beats hearing it. Open your mind to new scales and get used to how they sound. It's easy to "learn" that only a handful of scales work over any given progression. Remember that theory is based on music, music is not based on theory.

Listen to these and enjoy some less used scales:

 

It can be a little (or a lot) overwhelming thinking of all the possible combinations (I know I felt that way at first), so don't feel like you have to learn everything all at once.
You might look them all over, find one or a few that you really like, and play around with them.
And there's nothing wrong with that.
Lots of popular guitarists have favorite scales or patterns, and it can be part of their sound and their style.
But, if you can learn them all somehow, then I'll have to take some lessons from you
I'm not a great improviser, but I understand more now than I used to

 

I'm pretty impressed with myself for realizing that A minor = C Major = F lydian.

It's mostly helpful cuz like, if I'm playing over a track in A minor but I don't want to play up by the 12th fret (root 5th string), I know I can just play F lydian staring on the 8th fret. Plus I think lydian mode sounds cool. I assume there's a way to chain all the modes together and play huge extended runs all the way up and down the next but I haven't figured out how to do that yet.

Sometimes I just play whatever I hear in my head. I don't even think about scales or modes or anything, I just use relative pitch from one note to the next. Sometimes it goes really well and sometimes I miss notes like crazy But when I stop doing that I have to think about where I am and then what scales to use again.

 

I'm pretty impressed with myself for realizing that A minor = C Major = F lydian.

Good, now... quickly, go get all of those guitar scale books you own which show you different patterns for Lydian, Locrian, Aeolean, etc. and burn them. The 7 modal scales use the same 7 patterns, you only need to change which note you use as the root.

 

If you got the chance to record yourself while playing, do that. You might find that playing whatever you hear in your head is a lot more interesting to listen back to than well executed scale runs.

 

It's more melodic and harmonious sometimes, but only good for a while, for hooks, etc.

It's slower but I try to make it a good match for what the backing track is doing.

It's the same sort of stuff I sing to myself when I hear a song or something in my head. I can't really sing shredding scale runs :lol:

 

Now, just to twist your brain in a slightly different direction...

Sometimes when guitarists use the word 'modes' (Aeolian, Mixolydian, etc...), what they actually mean is the shape/pattern they are playing, which is different than playing a 'modal' song.
A modal song (something you can find examples of by composers like Bela Bartok, if I recall) can also be a very interesting thing.
Here is an example:

Take a simple familiar tune that you know, like Yankee Doodle, Happy Birthday (be careful, Happy Birthday starts on scale step 5, not on 1), Frere Jacque, etc...
Write down the numbers of the scale steps.

Yankee Doodle:
| 1 1 2 3 1 3 2 _ | 1 1 2 3 1 1 5 _ | 1 1 2 3 4 3 2 1 | 7 5 6 7 1 _ 1 _ |

Now, this is in major/Ionian, typically. But you can play this in any other mode and some will sound pretty cool or pretty strange.
For example, if you start a Lydian scale on 1, you have the same thing as Ionian but with a #4,
1 2 3 #4 5 6 7 1 (correct me if I'm wrong, it's been a while)

so you could play Yankee Doodle in Lydian
| 1 1 2 3 1 3 2 | 1 1 2 3 1 _ 5 _ | 1 1 2 3 #4 3 2 1 | 7 5 6 7 1 _ 1 _ |
What you have to keep in mind here, is that your chords will change from Ionian/major also:
instead of I ii iii IV V7 vi viidim
you have:
I II7 iii #ivdim V vi vii

try playing: I vi IV ii V7 I (major)
now playt he same thing in Lydian: I vi #iv II7 V I

lots of times in actual modal songs, you will find yourself using different dominant-functioning chords.
Instead of the Ionian/Major I V I, you might find yourself doing this in Lydian I II7 I.
It's different, but your ears can get used to these things.
If you are in Mixolydian, you might do this I bVII7 I (instead of I V I)
since in Mixolydian you have:
scale steps: 1 2 3 4 5 6 b7 1
and chords: I7 ii iiidim IV v vi bVII

it can get extra colorful if you add the correct 7ths to all of these chords also.

Try playing I vi IV ii V7 I
now in Mixolydian:
I7 vi IV ii v I7 (keep in mind, this v chord is minor in Mixolydian).
And, the dominant 7th on the I chord might sound confusing, so you could always leave it off.

Anywho, something you can play around with today, I have to get ready for work

I'll write out Yankee Doodle in the other modes if I remember later For the heck of it

 

Now, this is in major/Ionian, typically. But you can play this in any other mode and some will sound pretty cool or pretty strange.

I used to have some of my advanced students do this. Play the same scale degrees but on different scales. For example, I'd have them play "Frosty the Snowman" in minor. It would turn into "Frosty's Darkest Hour". It would still be clearly distinguishable as Frosty, but it would just be a little tweaked.

 

part 2:

Yankee Doodle in some common modes:

Ionian: | 1 1 2 3 1 3 2 _ | 1 1 2 3 1 _ 5 _ | 1 1 2 3 4 3 2 1 | 7 5 6 7 1 _ 1 _ |
Dorian: | 1 1 2 b3 1 b3 2 _ | 1 1 2 b3 1 _ 5 _ | 1 1 2 b3 4 b3 2 1 | 7 5 6 b7 1 _ 1 _ |
Phrygian: | 1 1 b2 b3 1 b3 b2 _ | 1 1 b2 b3 1 _ 5 _ | 1 1 b2 b3 4 b3 b2 1 | b7 5 b6 b7 1 _ 1 _ |
Lydian: | 1 1 2 3 1 3 2 _ | 1 1 2 3 1 _ 5 _ | 1 1 2 3 #4 3 2 1 | 7 5 6 7 1 _ 1 _ |
Mixolydian: | 1 1 2 3 1 3 2 _ | 1 1 2 3 1 _ 5 _ | 1 1 2 3 4 3 2 1 | b7 5 6 7 1 _ 1 _ |
Aeolian: | 1 1 2 b3 1 b3 2 _ | 1 1 2 b3 1 _ 5 _ | 1 1 2 b3 4 3 2 1 | b7 5 b6 b7 1 _ 1 _ |
Locrian: | 1 1 b2 b3 1 b3 b2 _ | 1 1 b2 b3 1 _ b5 _ | 1 1 b2 b3 4 b3 b2 1 | b7 b5 b6 b7 1 _ 1 _ |

Harmonic Minor: | 1 1 2 b3 1 b3 2 _ | 1 1 2 b3 1 _ 5 _ | 1 1 2 b3 4 b3 2 1 | 7 5 b6 7 1 _ 1 _ |


And of course, as was mentioned earlier, theory is a way to analyze music many times after it's been written, to try and figure out what's going on and what makes it sound the way it does.
This this can help you to imitate a composer's/musician's writing style, if you like.
Some people use chord formulas to write their music, some people just make things up on their instrument without thinking chords or scales, and some people probably use mixtures of the two or something completely different (i.e. labeling dice with notes or chords, rolling them and writing down what comes up, to see if they get something interesting.
Or something like some of Stravinsky's music where you can analyze it as note clusters (Set Theory - Pitch Classes),
every chromatic note gets a number 0 through 11 (t = 10 and e = 11), and instead of saying C Major chord, you would say 047, C minor 037, C7 047t, CMaj7 047e....
but it really gets cool when you make up things that don't have a name like 0235 (C D Eb F, treated as a chord),
or the Petrushka chord - two Major chords a tritone apart: CMajor with F# Major on top of it, or 02467t or 047 6t2 ....)

Okay, somebody stop me, haha.

 

lastsnare, thanks for posting that. I will have to read it a few more times and then might have questions

 

No problem, sorry I got way off-topic there and out of hand.
Those energy drinks first thing in the morning do some crazy things ... and now it's nap time ;P

 

the simple answer is, know your c major modes simply throw a g sharp instead of a g and you have harmonic minor, if it sounds good it will if it doesnt it doesent.

so any chord in c that does not contain g will work , so in the key of a minor or c major, you can change the key or feel by playing g sharp in the right place.