Don Firth, you're confusing two different eras. Monteverdi was indeed around in 1630 but by that time dominant sevenths and all kinds of weirdy chords were long established. Check out any elizabethan( 1500's) key board or lute music, there's loads available. (Including plenty of folk related material suitable for guitar). "Summer is icumen in" is much earlier,( c 1300? I'm not up on latest attempts to date the MS) and is indeed incredibly advanced for its time. Itis in six parts ( a four part round with a twoperson "burden" or bass riff). Speaking from memory, it doesnt use the full dominant 7 at any point but it certainly features the tritone(B-F) and the BDF "dissonant" triad. It is written in C major, and uses the mode andharmonies which still dominate folk/pop music to this day( in my neck of the woods anyway). It is so clever and so advanced for the period you'd be almost tempted to write it off as a forgery(or possibly brought in in a time machine or by little grey men from another galaxy). Mind you, I'm speaking from a viewpoint of a while ago. Probably modern comparative music research has thrown up more equivalent stuff from that era?