Knowing the chords in a key or scale is one thing, but knowing to pull out the right chords or notes at any given time is another beast. And unfortunately, this is very often requested from lead guitarists. But rest assured, with time, practice and diligence, anyone can definitely pull this off.
But to not discourage any young players out there, there are certainly tricks to choose right chord from any key or scale.
Firstly, use a guitar to demonstrate the interaction of the intervals, the chords in a scale and how to transform one scale into another. Take notice that in any given key, there are always 3 major and minor chords, and these chords usually relate to each other in one way or another.
For instance, take the key of A major, the major chords are A D and E, whose locations are 5th fret of the 6th string, 5th and 7th fret of the 5th string respectively. Using what we have learnt about intervals – the gaps between the notes, we can find other major chords of other scales. This pattern is applicable to any key as well. You need to find the major chords of the scale of C, no problem, just use the above pattern.
Now that we have found the major chords, the minor chords are also very easy to find. Still at the A of the key of A major, we move down 3 frets, there we find the F#, our first minor chord. From that position, drop down one string and we have a B then two frets to the right and we will have a C#, our last two minor chords.
This is a very powerful technique and will save you many hours of practice. Remember, we are learning the music theory of the guitar, so it is best to use a guitar as a tool to figure out the key.