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VIEW FULL LIVE VERSION : Scale Modes: help needed please.
Rockin John 07-01-2005, 06:25 AM Hello folks...
I have 2 small books that discuss scales for bass players. Each looks at modes, but both in a different way.
One book describes each mode as being the new sacle generated when starting on different points of the Major scale.
Starting with C Maj, for instance, Dorian begins on step 2 of the scale (=D) and ends an octave higher, also on D. He gives the new scale the name, D Dorian, because it begins on D. It has no shars or flats in the Key sig. All other modes he begins on the correct steps of the scale.
*****
The other book starts all the modes of C Maj on the note, C. The author adds accidentals on the staff as required to produce the pattern of whole / half notes that will make the Dorian mode. Thus his accidentals are 3b and 7b which equate to Eb and Bb for C Dorian.
My confusion arises mainly from the first example. Sticking to the C Maj example, is the scale the begins on the second step actually a variant of C Maj, called C Dorian? Or is it actually a D scale played without any sharps?
Naturally the same applies for all modes, not just Dorian.
Or am I just miles away from reality?
Thanks.
John
Richard Lindsey 07-01-2005, 07:44 AM No, the scale D E F G A B C D (and every mode is a scale) is always D dorian. It is never C dorian, though I can see how you could get confused that way.
This is one of the reasons why I think the modes are best taught in the second of the two ways you mention.
Richard Lindsey 07-01-2005, 09:41 AM Rockin John, here's an interesting site:
http://www.dolmetsch.com/musictheory25.htm
One thing to remember about using the "degrees of the major scale" approach to derive modes: Although you can and do derive the modes that way, modes are also in a way tonalities/modalities of their own. They don't *necessarily* always have any connection to the major tonality that provides the "source scale". Thus, you can have a piece that is in D dorian. The student might be tempted to think, well, doesn't that mean it's really in C major, since D dorian is " the second mode of C major." But the answer is no. D dorian, as a tonal center, is different from the key of C major, even though the D dorian mode has the same component notes as a C major scale. You're probably familiar with the famous Miles Davis tune "So What." The harmony is basically D dorian/Eb dorian/D dorian. Not, it is important to note, C major/Db major/C major. Much rock, folk, and ethnic music is actually more modal than it is strictly major/minor.
Rockin John 07-01-2005, 11:03 AM I just knew I was opening-up a can of worms with this. ;)
There appears to be absolutely nothing about learning musical theory that's that's in any way straight forward or simple.
Phew........
I can see that both are methods end up with the same result.
Wait, are you saying that any Dorian scale is called D Dorian...even if it starts on (say) Gb?
Here's another thought. I conjectured last night that Cm7 must be the root cord of D Dorian because of the 3b and the 7b. Is that correct? No, it can't be.........
Richard, thanks for the link. I will follw it when my head's clearer. :eek:
John
Richard Lindsey 07-01-2005, 11:15 AM Wait, are you saying that any Dorian scale is called D Dorian...even if it starts on (say) Gb?
No no no. I'm saying that D E F G A B C D is always the D dorian mode, irrespective of whether it was derived from the C major scale or independently generated. It's never the C dorian mode, as you wondered in your initial post.
A mode is nothing but a set of pitches at specified intervals from a specified starting point. A C major scale, or C ionian, is just a certain set of pitches at certain distances from C. A C dorian mode is a different set of pitches at different distances from C. The note that you start from in figuring the mode is the note that gives its name to the mode.
If W = whole step (2 frets) and H = half step (1 fret), then the pattern for a major scale (ionian mode) is
WWHWWWH
Start with note X, pile the intervals on, and you have X major, or X ionian.
The pattern for dorian is
WHWWWHW
Start with note X, pile on the intervals, and you have X dorian.
Richard Lindsey 07-01-2005, 11:23 AM Here's another thought. I conjectured last night that Cm7 must be the root cord of D Dorian because of the 3b and the 7b. Is that correct? No, it can't be.........
Well, no, it's not correct.
"Root chord" is not a concept I use much myself, and I tend to derive chords from scales more than the other way around, though in the Great Scheme of Things they're kinda the same thing anyway. I suppose you could think of Dm7 as the basic chord of D dorian. (Though you should not assume that Dm7 always implies D dorian, because the notes you would play with that chord depend on the harmonic context in which Dm7 occurs.) If you built a stack of thirds from D in D dorian, you would get a Dm7, then the 9th (E), then the 11th (G), and finally the 13th (B). So I guess you could say the chord of the dorian mode is Dm11/13, if you want to look at it that way.
triggert 07-01-2005, 01:34 PM If W = whole step (2 frets) and H = half step (1 fret), then the pattern for a major scale (ionian mode) is
WWHWWWH
Start with note X, pile the intervals on, and you have X major, or X ionian.
The pattern for dorian is
WHWWWHW
Start with note X, pile on the intervals, and you have X dorian.
Gonna jump in on this and ask a quick question, I'm starting in on some theory too and all these scales are confusing. That webpage you posted will help a lot it looks like. So if I use the pattern WWHWWWH starting with G - GABCDEF#g - that is going to be my G Major/Ionian scale, right? Then if I use the pattern WHWWWHW with G - GABbCDEFg - thats gonna be my G Dorian scale?
Trigg
Richard Lindsey 07-01-2005, 02:42 PM So if I use the pattern WWHWWWH starting with G - GABCDEF#g - that is going to be my G Major/Ionian scale, right? Then if I use the pattern WHWWWHW with G - GABbCDEFg - thats gonna be my G Dorian scale?
Yes, exactly.
triggert 07-01-2005, 03:19 PM OK, so building the scale is simple enough but how do you figure out which chord(s) you can play it with? I had read that the first, third, fifth, and seventh notes are the chord tones. So using the the G Major and G Dorian I'd come up with - GBDF# and GBbDF would be the chords, correct? How could you figure out the names for those 2 chords?
Thanks,
Trigg
Richard Lindsey 07-01-2005, 04:13 PM How could you figure out the names for those 2 chords?
Well ... by learning chord construction. I can't put my hand on one right now, but I'm sure there are lots of good sites that teach that. Check the stickies at the top of this forum, for starters.
BTW, those two chords are Gmaj7 and Gmin7.
Something else you can do is try the 1-3-5-7 thing with *all* the notes in the mode. This gives you the 7th chords that occur "naturally" within the mode. So in G ionian/major, you have
G B D F#
A C E G
B D F# A
and so forth. In G dorian, you have
G Bb D F
A C E G
Bb D F A
and so forth.
triggert 07-02-2005, 02:51 AM BTW, those two chords are Gmaj7 and Gmin7.
So if I was to do the samething but change the scale then those scales would be Xmaj7 and Xmin7? I have a program called EasyChord and I was looking through at all the maj7 scales and some of them don't match up from what I can tell. For example, according to the program a Bmaj7 chord would be - B F# A# D# but if I use the chord tones from the B major scale I get - B D# F# A#
So what am I missing here, this is the only thing really throwing me off...
Trigg
JimmyM 07-02-2005, 03:45 AM What you're missing is a new vocabulary word..."inversion." It's the same chord with all the same notes, just a different order. I don't know EasyChord but it sounds like it's more guitar and bass oriented. If you played a Bmaj7 chord on a bass it would go B F# A# D#. But if you put the Bmaj7 chord together yourself, you would get the notes in the regulation order of B D# F# A#. It's the exact same chord, just with the notes in a different order, or an inversion.
Richard Lindsey 07-02-2005, 09:16 AM So if I was to do the samething but change the scale then those scales would be Xmaj7 and Xmin7? I have a program called EasyChord and I was looking through at all the maj7 scales and some of them don't match up from what I can tell. For example, according to the program a Bmaj7 chord would be - B F# A# D# but if I use the chord tones from the B major scale I get - B D# F# A#
So what am I missing here, this is the only thing really throwing me off...
Trigg
JimmyM is right on the money with inversions. That's what it's about--same notes, different order. On a chordal instrument--which bass isn't to the same degree as piano or guitar--you arrange the notes in a way that achieves the sound you're trying to get at a given time. As long as the components are the same, the chords are the same (though there are a couple wrinkles to this, in that there are some chords with different names that have the saem notes, but don't worry about that right now).
And don't confuse scales with chords. Gmaj7 and Gmin7 are chords, not scales. A given chord can often be derived from (or occur naturally within) more than one scale. For example, if you do the 7th chord thing with the 1st note of G ionian/major, you get a Gmaj7 chord--but you also get the same chord (G B D F#) if you build a 7th chord off the 1st note of G lydian; the 4th note of D ionian or mixolydian; the 5th note of C lydian; the 3rd note of E phrygian; the 6th note of B aeolian; and so on.
triggert 07-02-2005, 01:17 PM What you're missing is a new vocabulary word..."inversion."
Thats exactly what I was missing. I'd seen the word around but sometimes it just helps when someone breaks it down for you. Everythings slowly starting to fall together.
For example, if you do the 7th chord thing with the 1st note of G ionian/major, you get a Gmaj7 chord--but you also get the same chord (G B D F#) if you build a 7th chord off the 1st note of G lydian; the 4th note of D ionian or mixolydian; the 5th note of C lydian; the 3rd note of E phrygian; the 6th note of B aeolian; and so on.
Even though you can get the same chord out of other scales the name doesn't change, is it still just a Gmaj7?
Thanks,
Trigg
Richard Lindsey 07-02-2005, 04:38 PM Even though you can get the same chord out of other scales the name doesn't change, is it still just a Gmaj7?
Yes. G B D F# is always a Gmaj7 chord.
Rockin John 07-03-2005, 04:28 PM Unfortunately life has kept me away from the computer (and, therefore, TB). But I've been sticking the the music learning and have yet another (naive?) question to put to the learned ones:-
If I decide to play a whole song in the Lydian Mode of E Maj, why am I not actually playing in B Maj? E Maj has 4 sharps, the Lydian Mode sharpens the forth degree of the Maj scale (so in E Maj, the A now = A#), so the whole song now has F# C# G# D# A# = B Maj.
That just has to be rubbish but I honestly cannot figure out why :ninja:
John
Richard Lindsey 07-03-2005, 04:41 PM Unfortunately life has kept me away from the computer (and, therefore, TB). But I've been sticking the the music learning and have yet another (naive?) question to put to the learned ones:-
If I decide to play a whole song in the Lydian Mode of E Maj, why am I not actually playing in B Maj? E Maj has 4 sharps, the Lydian Mode sharpens the forth degree of the Maj scale (so in E Maj, the A now = A#), so the whole song now has F# C# G# D# A# = B Maj.
That just has to be rubbish but I honestly cannot figure out why :ninja:
John
Because to say that you're playing "in" a key mode means not only that you're specifying a certain set of pitches, *but also that certain pitches will be more harmonically fundamental to the piece than others*. For example, to say that you're playing in E lydian does not *only* mean that you're using the set of notes E F# G# A# B C# D#. It does mean that, but it also, and perhaps more importantly, means that E--not B--functions as a tonal center for the piece.
Suppose your piece starts in E, ends in E, goes to E at most cadences, and generally spends most of its time landing on E when the chips are down. Then E can really be said to be the tonal center of what you're doing. Thus, however you describe the mode of what you're playing, it should be some kind of E mode. If you were using the same set of notes, but instead B appeared to be acting as the tonal center, you would describe what you're doing as some kind of B mode/key.
Rockin John 07-04-2005, 02:24 PM Thanks.
That sounds fair as far as I understand.
So for E, the Key Sig would still be 4 sharps?
But every A would be accidentally # (unless A nat was required in a given bar(s) )?
From that the musician would understand that the Key (tone centre?) was E; not E Maj, but the Lydian mode of E?
Is that correct?
PS.
What are cadences?
:D
Thanks.
John
ehiunno 07-04-2005, 10:23 PM Unfortunately life has kept me away from the computer (and, therefore, TB). But I've been sticking the the music learning and have yet another (naive?) question to put to the learned ones:-
If I decide to play a whole song in the Lydian Mode of E Maj, why am I not actually playing in B Maj? E Maj has 4 sharps, the Lydian Mode sharpens the forth degree of the Maj scale (so in E Maj, the A now = A#), so the whole song now has F# C# G# D# A# = B Maj.
That just has to be rubbish but I honestly cannot figure out why :ninja:
John
i think this is the reason most people get confused when it comes to modes. though the lydian mode "sharps the 4th of the scale" that is only because when you base the scale off of the 4th scale degree, yet play the notes in the major scale of that key, the lydian relates to the major scale as having a sharp 4.
if your playing in E (E,F#,G#,A,B,C#,D#,E) and you decide to play in the lydian mode of E major your simply starting on the 4 and playing those exact same notes (A,B,C#,D#,E,F#,G#,A). to reiterate, if your playing a song in any mode of a major key your still playing the same notes as in the major key.
NOTE: the next part is confusing and complicated, just a warning. if you have any questions feel free to ask, if i made any mistakes please please correct me!
where this becomes confusing is when you say E lydian, which would in theory be E major with a raised 4, but a lot of people mistakenly use that term to describe the lydian mode of E major which would be A lydian. kind of how C major and A minor(aolean, spelled it wrong) are the same key signature. minor is simply thesame notes of the major played off of the 6th scale degress just like lydian is played off of the 4. now you woudlnt call a minor scale with no sharps and flats C minor would you? of corse not! thats why you would call a lydian mode of the E major scale A lydian. now if you want to write a song that is truely in E lydian you would be using the tonal centre of B major and YES! YOU WOUDL BE PLAYING IN THE KEY OF B MAJOR!
i hope i havent just covered a ot of stuff you already understand and said what everyone else has said in another wordind, but i hope it helps
Richard Lindsey 07-05-2005, 08:36 AM where this becomes confusing is when you say E lydian, which would in theory be E major with a raised 4, but a lot of people mistakenly use that term to describe the lydian mode of E major which would be A lydian. kind of how C major and A minor(aolean, spelled it wrong) are the same key signature. minor is simply thesame notes of the major played off of the 6th scale degress just like lydian is played off of the 4. now you woudlnt call a minor scale with no sharps and flats C minor would you? of corse not! thats why you would call a lydian mode of the E major scale A lydian. now if you want to write a song that is truely in E lydian you would be using the tonal centre of B major and YES! YOU WOUDL BE PLAYING IN THE KEY OF B MAJOR!
I think there's some confusion here. Just my $0.02, but I think it's a little misleading to talk about A lydian as being the "lydian mode of E major." I know it's done, but I think that's one of the things that screw people up about modes. Call it the fourth mode of E major maybe. Calling it the lydian mode of E major makes might make some of us (well, me anyway) wonder if you mean E lydian. My own bias--and I admit to having strong opinions about this--is that the whole "modes of the major scale" concept, building them off successive degrees of the major scale, may be more trouble than it's worth.
And no no no, if you are writing a song that is "truly in E lydian," you are NOT using the tonal center of B major! You are NOT in B major, even though you are using the same notes that are in B major. That was the whole point of my earlier post, and again, this is part of the reason why attaching modes inextricably to degrees of the major scale can mess up understanding them IMO.
As I said, when you say that a piece is "in" some key or mode, you are specifying two things. (1) The tonal center, or the note that serves as the tonic or whatever. This note gives its name to the key or mode. Thus in G major, the tonal center is G; in A minor, it's A; and in E lydian, it's E. (2) The set of notes that occur naturally in that key or mode. In D dorian, it's D E F G A B C; in E lydian, it's E F# G# A# B C# D#.
Now, you cannot meaningfully be "in" a key *that is not also the tonal center*. Thus, if your piece is truly in E lydian, you cannot, by definition, be in B major--simply because B is not the tonal center, E is. Even though the component notes are the same. Similarly, a piece like "So What" (my favorite example, just because it's so clear and simple) is in D dorian and Eb dorian. It is NOT in C and Db. Even though the component notes are the same.
It's best to get away from the idea that modes *have* to be attached at the hip to major scales in some way. You do find them naturally occurring in standard major-minor harmony, but they're not limited to that. They exist as their own musical entities. A mode is not just a derivative of a scale, regardless of how it may have started. It *is* a scale in its own right. That's why I think it's important to learn them in a parallel fashion, that is, from the same starting point. That's how you're really gonna hear how they differ from each other.
Richard Lindsey 07-05-2005, 08:50 AM Thanks.
So for E, the Key Sig would still be 4 sharps?
But every A would be accidentally # (unless A nat was required in a given bar(s) )?
From that the musician would understand that the Key (tone centre?) was E; not E Maj, but the Lydian mode of E?
That's one way of doing it, and a fairly common one. Much rock, pop, and folk is, at least in spots, as much modal as conventionally tonal. Think of any of 8 zillion songs that clearly have a tonal center of D, but they cycle around D C G D, with never an A to be found. It's common to use a D major key sig and notate all the C naturals as accidentals. Sometimes in jazz, when the harmony gets screwy and not easily explicable in standard functional terms, the composer dispenses with key sigs altogether and just notates all sharps and flats as accidentals.
ehiunno 07-06-2005, 04:33 PM I think there's some confusion here. Just my $0.02, but I think it's a little misleading to talk about A lydian as being the "lydian mode of E major." I know it's done, but I think that's one of the things that screw people up about modes. Call it the fourth mode of E major maybe. Calling it the lydian mode of E major makes might make some of us (well, me anyway) wonder if you mean E lydian. My own bias--and I admit to having strong opinions about this--is that the whole "modes of the major scale" concept, building them off successive degrees of the major scale, may be more trouble than it's worth.
And no no no, if you are writing a song that is "truly in E lydian," you are NOT using the tonal center of B major! You are NOT in B major, even though you are using the same notes that are in B major. That was the whole point of my earlier post, and again, this is part of the reason why attaching modes inextricably to degrees of the major scale can mess up understanding them IMO.
As I said, when you say that a piece is "in" some key or mode, you are specifying two things. (1) The tonal center, or the note that serves as the tonic or whatever. This note gives its name to the key or mode. Thus in G major, the tonal center is G; in A minor, it's A; and in E lydian, it's E. (2) The set of notes that occur naturally in that key or mode. In D dorian, it's D E F G A B C; in E lydian, it's E F# G# A# B C# D#.
Now, you cannot meaningfully be "in" a key *that is not also the tonal center*. Thus, if your piece is truly in E lydian, you cannot, by definition, be in B major--simply because B is not the tonal center, E is. Even though the component notes are the same. Similarly, a piece like "So What" (my favorite example, just because it's so clear and simple) is in D dorian and Eb dorian. It is NOT in C and Db. Even though the component notes are the same.
It's best to get away from the idea that modes *have* to be attached at the hip to major scales in some way. You do find them naturally occurring in standard major-minor harmony, but they're not limited to that. They exist as their own musical entities. A mode is not just a derivative of a scale, regardless of how it may have started. It *is* a scale in its own right. That's why I think it's important to learn them in a parallel fashion, that is, from the same starting point. That's how you're really gonna hear how they differ from each other.
thats another good way to put it, when i think about modes im usually thinking in roman numeral dictation where if your playing a ii V7 I in C major your playing D dorian, G mixolydian, and C ionian, all while playing the same notes and key centre of C maj. if a song was truely in E lydian you would be still be playing the I as E and not B so i see what yoru saying, even though the I would be the lydian mode, which would confuse the living crap out of me and it woudl be a lot simpler to call it the IV of Bmaj, even though thats not what hes trying to do
bassplayer2106 07-06-2005, 04:53 PM Hi there are two types of mode,those derived from a particular major scale and those relating back to a scale. C dorian ( C,D,Eb,F,G,Ab,Bb,c ) would actually relate back to the Bb major scale it being the second of that particular scale.The dorian mode of C major is D dorian (D,E,F,G,A,B,C) the second note of that scale.Basically you are just playing the original major scale starting a tone up on the second or a minor scale with a major 6th. It can be very confusing learning from books.You only have to get one thing wrong without knowing it and then everything else afterwards can be wrong.You then spend ages unlearning it all and relearning.My strong advice is to get a good teacher.I learned the hard way.
Rockin John 07-07-2005, 04:32 PM In the few days over which I've being trying to learn about Modes, my personal preference is falling towards regarding Modes as scales in their own right, rather than them being attached to a Major scale. I personally find the concept too confusing.
If E Lydian has (effectively) the same Key Sig as B Maj, I'll just have to learn to live with that.
Richard, I found your reply to ehiunno very enlightening. It answered questions that I didn't even know I needed to ask. Brilliant. :D
John
Richard Lindsey 07-07-2005, 06:48 PM In the few days over which I've being trying to learn about Modes, my personal preference is falling towards regarding Modes as scales in their own right, rather than them being attached to a Major scale. I personally find the concept too confusing.
Glad to be of help, mate.
Regarding the modes, it doesn't have to be totally an either-or thing. It's not at all bad to know how the modes occur as derivatives of a major scale; I just don't think it's helpful to think of them as inseparably joined at the hip.
But you really don't always have to think of playing a different mode with every chord, at least not in functional tonal harmonic settings where all you're really doing is executing a "frame shift" on a single major scale. If you're playing | C Dm | G C |, that's clearly a diatonic progression in C. You can think of yourself as being in C throughout, and just select different "prefer" and "avoid" notes for each chord. (Note: this does not mean just noodling in C--quite the opposite.) Often, you really don't need to posit C ionian, D dorian, G mixolydian, C ionian, though of course you can if you want. (At faster tempos, I don't want to, and probably can't, work that hard for fast-moving chords!)
Rockin John 07-08-2005, 06:37 AM Oh dear, I'm getting lost again......
Richard said:
If you're playing | C Dm | G C |, that's clearly a diatonic progression in C. You can think of yourself as being in C throughout, and just select different "prefer" and "avoid" notes for each chord. (Note: this does not mean just noodling in C--quite the opposite.) Often, you really don't need to posit C ionian, D dorian, G mixolydian, C ionian, though of course you can if you want.
Please (he asked feeling rather foolish) what is a diatonic progression? :eek:
For that diatonic progression, please explain why you chose D dorian for the Dm, and G mixolydian for G. I don't understand that, I'm affraid. (I have made the presumption that C is ionian because it is the tonic or tonal centre as you have outlined previously?)
And, do you mean that each new chord being used within an existing scale can be regarded being part of another scale / part of a scale named after the chord? That doesn't sound right, does it?
Thanks.
John
Richard Lindsey 07-08-2005, 08:30 AM Oh dear, I'm getting lost again......
Richard said:
Please (he asked feeling rather foolish) what is a diatonic progression? :eek:
For that diatonic progression, please explain why you chose D dorian for the Dm, and G mixolydian for G. I don't understand that, I'm affraid. (I have made the presumption that C is ionian because it is the tonic or tonal centre as you have outlined previously?)
And, do you mean that each new chord being used within an existing scale can be regarded being part of another scale / part of a scale named after the chord? That doesn't sound right, does it?
Thanks.
John
All I mean by a diatonic progression is one that contains only chords made of notes that occur "naturally" in the specified original key, without accidentals. Say you're in C major. That means the "naturally occurring" notes are C D E F G A B. So in the progression C Dm G C, the chords contain *only* various combinations of those seven notes (C E G, D F A, G B D, C E G). No jokers in the deck--no Bb, no D#, nothing outside the original set.
One way of approaching the idea of how to play over different chords is to posit that when you go to a different chord, you're playing a different mode of the parent major scale. This is the consequence of the "modes of the major scale" approach. The dorian mode is the mode you get if you take the notes of the C major scale and do a "frame shift" (as with DNA copying) and start with the second degree of the scale, namely D. If you do the frame shift to the fifth degree, G, you get G mixolydian. That's where the modes I mentioned come from.
Now, this approach works, if you can keep up (bebop at 300 bpm, anyone?), but I personally have come to find it unnecessary and needlessly complex in many cases. In no analysis of standard functional harmony that I'm aware of is it necessary to assume that you're going to a different harmonic structure every time you change chords. For stuff that clearly fits within a key and functions in something like a "standard" way, I think it's easier to use the parent key/tonality approach (see some of Chris Fitzgerald's theory stickies above), and just make selections from within the parent scale on the basis of where you are at the moment. You don't necessarily have to think that you're going to a different mode of the major scale.
For example, if you take the simple C major progression I mentioned, on the C chord your chord tones are the 1 3 5 of the C scale, or C E G; these then are your "preferred" notes, with the C being often, but not always, the strongest. When you hit the Dm, you can simply select different preferred notes from within the same set--namely, D F A, your chord tones. Same with G--G B D. The idea is that you think of yourself as being within a single tonality, but you make different selections from your "pool" of notes to convey the harmonic and melodic movement that's happening. (This is not to say that you can use only the preferred notes with a given chord, it's just to say that the preferred chordal notes are your basis, your starting point. Clearly players often use nonchordal, and sometimes nondiatonic notes in their lines. But that's another subject.)
One possible drawback of this approach is that it could conceivably be interpreted to mean that all you have to do is wander around in C through the whole thing. But that's not what I mean at all. No random wandering--*intelligent selection* is the key. You can't just play any old thing within the set C D E F G A B. Think of it as something like a phone number. You only have 10 possible digits, but you can select from them intelligently to create distinct phone numbers. And if you dial wrong, you don't get the person you were trying to call.
Now, obviously, this doesn't explain anything like the whole picture with regard to playing over or under chords (not that I think I'm capable of commanding the "whole picture," whatever that may be, anyway). This is only a nibble off the end of the enchilada. There are times--many times, in the various types of popular music--where you don't *want* to play purely diatonically, where the music in fact demands that you don't (cf. blues). Also, there are many many tunes in which there isn't one and only one parent tonality through out the piece; many tunes have more than one.
Let's leave those things for another day, or for better explicators than I am. I'll just say that in such cases, a parent tonality may be hard to identify or absent entirely or not even an applicable concept, and the "different mode for different chord" approach is often useful and even preferable.
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