Here is a 26min video that explains what modes are, why you might need to use them, where they are and how to identify and remember the box positions for each. This video guitar lesson compliments the information on the remainder of this page.
The modal system can seem frightening, complex, or just plain pointless!
I am therefore starting with a simple example, one bar of G, followed by a bar of F and C:
Referring to the page 'What chords to use?', it could be said that this progression is in the key of C major (ie these chords can all be derived from the C major scale).
However, by improvising over the track (or even simply looking at it) we can establish that the 'tonal centre' is G (ie the progression 'rests' on the chord of G major and a G note 'sounds strongest' throughout the progression).
Many players may regard this piece as being in the key of G major for this reason.
However, this cannot be so - because if we play the notes of the G major scale over the progression, slowly and listening carefully, we can hear that it doesn't quite fit.
The notes of G major are G, A, B, C, D, E, F#, G.
It is the seventh note of F# that doesn't fit over the F chord.
So it is the notes of the C major scale that fit - C, D, E, F, G, A, B, C - but we need to take into account that the tonal centre of the progression is G.
What we therefore end up with is G, A, B, C, D, E, F, G - the G mixolydian mode scale.
This demonstrates the practicality and simplicity of the modal system.
The above process involved deriving a G scale from the C major scale, and this is what the modal system does. It takes each note of a major scale and regards that note as the tonal centre. Here is what you get, using the C major scale: