Sometime around 1400, a Great Vowel Shift occurred. It resulted in this incomprehensible yet oddly fascinating chart. Oh, and it also meant people started to speak English in a completely new way. (Via Black Market Kidneys.)
Apparently vowels are shifting again in inland northern U.S. cities, resulting in the spread of that rattly “a” sound — and if you’re not from the Northeast, but have visited or moved here, you’ll know exactly what I’m talking about — to words that don’t even contain the letter “a.” “Block,” for instance.
And contrary to popular belief, says linguist William Labov, streamlined TV speech hasn’t resulted in a more uniform accent country-wide. In fact, accents are diverging more than ever before. Which will only make it harder to understand each other in 20 years, when we’re all living together in the Midwest — on the coastline.
See also “An amateur dialectician’s revenge manual.”