...for example, if you want to stay in the Dorian mode, you can alter your scalar playing whenever the band change chords. So the band stay in the one key, but you exploit the chord change to retain the Dorian signature (ie you effectively start playing in a different key).
Can you give me a quick example of a chord progression and a set of corresponding notes play from staying in the dorian mode?
This stuff is beginning to come clearer, but I need a concrete example for my sanity
In order to really understand how modes work, you need to understand the concept of Tonality. But beyond that:
Let's take D Dorian as an example. The governing principle of the mode (what gives it its particular flavour and distinguishes from other modes) is the major sixth above the final or root, the minor 3rd and the natural (or flattend 7th if you're thinking in tonal terms).
So it follows that common chords in Dorian (in this example Dorian on D) are:
G maj (this is where the major 6th comes into play and differs from a key - ie chord 4 in a minor key is always minor, but in Dorian it's always major)
C Maj (this is chord 7 and it's major; in a minor key it would be diminished)
A Min (Chord 5; in a minor this would be Major (we'd have a C#) but in Dorian, chord 5 is always minor)
E Min (this is where the major 6th comes into play; chord 2 in a minor key is always diminshed)
B Diminished (chord 6 is dimished in dorian, whereas in a minor key it would be major)
So, all the primary chords are there as in any 'key', but note how the governing principle of the mode alters the way the harmony works.
Indeed, to put it another way, many ppl confuse D minor with D Dorian. But they are fundamentally different - consider the following:
As we know F Major is the relative major of D Minor
How can we tell whether we're in F maj or D min? In the KEY of D minor we'd see C#s (leading note) and Bbs and indeed a gravitation towards D.
Now how can we tell if we're in D minor or D Dorian? Simple. No C#s (since modes do not need sharpened leading notes in order to modulate - because they don't modulate! And they don't modulate because modulation is a tonal concept and construct), and this means that chord 5 in Dorian is always minor (whereas it is always must be major in a KEY). And we wouldn't see Bbs either, because of the major 6th flavour of the mode.