hi Terry, I guess what I actually meant to say was that there is already such a wealth of music theory knowledge already available on the web, that rather than taking up undue bandwidth, providing links to existing internet sites on the subject might be better. Joe Water's 'Secrets to Chords & Progressions' tutuorial series located on his PSR Tutorial website is a fine example of this. I particularly like chapter 30 about inserting neigbhoring 'chromatic approach' type chords to the basic I-vi- ii- V progression.
One of the reasons pro musicians prefer to add 'chord substitions' (chromatic approach, tri tone subs, etc) to this progression is that after awhile, hearing this chord progression played over and over gets BORING. Chord substituions can add spice and a more contemporary or sophisticated sound to an otherwise basic standard progression. Experminenting with chord substitions & voicings is one KEY ingredient to sounding like a pro vs amatuer.
The I-vi-ii-V7-I progression is used extensively on so many popular songs over the years. Just to name a few songs: Let's Call the Whole Thing Off, Heart and Soul, Sleigh Ride, Hey There, Why Do Fools Fall in Love, Try to Remember, Have yourself a Merry Christmas, I Love YOu For Sentimental Reasons, Put Your Head on My Shoulder, Daddy's Home, Telstar, Breaking Up is Hard to Do, Penny Lane, Don't It Make My Brown Eyes Blue. One of the reasons this has become such a standard progression, is that the interval between each chord is up a perfect 4th. This internval provides the strongest harmonic movement in western music, with the V7 -I final cadence producing the strongest movement of all.
In fact, whenever you you play a domininant 7th chord, like G7, it 'begs' for you to play the I chord (C) in order to resolve the sound of instability in the G7? If you haven't already tried this, please go to the keyboard NOW and play a G7 chord. Play that G7 chord a few times. Doesn't it seem to be wanting you to follow that chord with a C maj chord? Please 'report back' after you've tried this out yourself, and tell me if you agree with me or not. Scott