Originally Posted by Joe-Bob
Locrian was never a mode. There are no pre-20th Century compositions that even use it. It is purely a theoretical construct, and not a result of analysis of actual compositional practice.
Originally, there were (are) 4 modes in two versions each. Compositions that went past their hexatonic ranges technically borrowed from another mode(s) to do so. It can often be very difficult to determine the actual mode of a Renaissance composition; one must often search to find the final structural cadence in order to find the correct answer. Increasingly over time, ficta was used to lend stronger cadences and other harmonic emphasis. This gradually became the current system of tonality, in which there are only two modes: major and minor.
This happened around 1620-1650, but it was a shift over time, and not a sudden change. Much of Bach's work involved re-setting old modal melodies to "modern" harmonies. (This explains many of the oddities that can be found where he breaks "his own rules".)
An example of just such a melody is O Sacred Head Now Wounded; a Phrygian melody set to tonal harmony. Old Phrygian pieces commonly had cadences on A and C, (not on Bb = the tritone), and in that piece, Bach acknowledges this with an unmistakable cadence on A major in a piece he places in C.
Whoa dude! You guys sure know how to party! Lol
Seriously, Joe-Bob, would you be kind enough to post a link to the Bach piece in C major with the A major cadence?
I don't think I've ever heard that but it sounds pretty cool.