Here's a UK-based site that does a nice job of "tracing" (ha! get it?) the functional and artistic changes to the gothic window style from Perpendicular (mostly UK) through the high gothic period. There are some nice modern line drawings that help illustrate the styles that evolved over hundreds of years and great distances apart from each other. For the architecture geek: Lookingatbuildings.org.
Trying to exercise the medieval art history muscles in my brain.