Common (also called the Common Tongue or Common Speech) is a widespread language descended from the largely unwritten, non-standard form of Thulian spoken by the lower classes and conquered subject peoples of the Thulian Empire. In the years following the collapse of the Empire, Common came to be used as a trade and diplomatic language between the various city-states and nations that arose from the ashes. Of course, Common shows much local variation in orthography and vocabulary, which is why there is a 1 in 6 chance that a speaker of one dialect may either misunderstand the speaker of another or fail to convey his own meaning, if the speaker has not familiarized himself with the new dialect beforehand. Nevertheless, knowledge of any dialect of Common is usually sufficient to get by in all but the most remote or far-flung regions previously governed by the Thulian Empire (and in many regions outside it).
EDIT: I had thought I'd set this post to publish tomorrow morning (April 4, 2011), since Sundays are excluded from the A to Z Blogging Challenge. Since I erred and Monday's post appeared today, I will delay "D is for Demon" until Tuesday (April 5, 2011).