I have checked out Justin's site and found it to be comprehensive and informative. I have always felt that learning about music and especially music theory applied to the guitar, is helpful in finding your own unique voice on the instrument and expanding your creative horizons. Along with his insight into teaching and his fantastic abilities on the instrument, Justin has created a powerful go-to-place for anyone interested in exploring the instrument to their potential. Just don't hurt yourself.
Guitars have been played since the Renaissance era, after descending from the ancient Greek instrument, the kithara. Or, at least, that's how the theory goes: the only real proof for this is the similarities between the Greek word and the Spanish word, quitarra. Early guitars often had four strings, evolving into the six-string version we know today in the late 1600s. Anton Stradivari, the famous violin-maker, also had a hand in making guitars. There's now only one Stradivarius guitar left in existence.