#2
If you know all the chords of the progression, you could write down all the notes used in those chords, count the number of sharps/flats and find the key signature which has the same amount of sharps and flats. For example, let's say we have a chord progression which is D, Em, G and A. If you write down the notes of all those chords:

D major: D F# A
E minor: E G B
G major: G B D
A major: A C# E

You'll see that there are two sharps, F# and C#. Knowing the circle of fifths might help, because those two sharps tell you that the key is D major, since it's the only key with two sharps in its key signature. This method does however not always work, since if the A major chord would've been left out of the progression, we would've been left with one sharp (F#) making the key G major. I suggest learning some music theory, it will all become clear.

Sometimes I can hear what key it is just by playing the chord progression, but that's a matter of training your ears.

Improvising around those chords can be done in a multitude of ways, and I'll leave that answer for someone who knows enough about the subject to explain properly.
Last edited by Cerasti at May 18, 2008,
#4
UG is not the only place where you can find information about music theory; I would recommend learning from multiple sources. Start with the basics -- learn where the notes are on the fretboard, what intervals are, the major scale, and basic triads. Don't move on until you've grasped the concepts of each part.

This is a nice page. Scroll down a bit and work your way through the pages, at your own pace. Picking up a book on the subject could also help.
#5
Ears training.
You can also just look at the Key signature on the left hand side.

The basic triads will help . You'll hear the difference between maj
min and dim.

It won't help you on all song..but it'll help.
If you know the invertal of a diatonic sytem and I,IV,V =maj
II,III,VI= min

Knowing that the Dominate( V chord) are mostly played with a b7
example.. D7, you can pretty much figure out that it's in the key
of G

Example if you hear or see Dmaj and C#min...being familar with
the invertals...you can reason that the Dmaj is the IV and C#min is the III.
This will be in the key of A

It won't be in the key of Dmaj becuase the VII is a diminished.
To be in the key of D.... it'll be Dmaj, C#dim

little stuff like....
Last edited by Ordinary at May 18, 2008,
#6
I'd recommend picking up a copy of The Complete Idiot's Guide to Music Theory if you want to get into learning theory. It taught me quite a bit.