About - What is the Rosetta Stone?

http://ancienthistory.about.com/cs/egypt/p/rosettastone.htm

The above hyperlink goes to the site About, a general information site that discusses topics such as Arts and Literature, History, Science, as well as resources for students.  The page dedicated to the Rosetta stone gives general information about the artifact, describing what it is, where it was discovered, its location, the languages on it, and how it was deciphered.  It also discusses the contents that are to be found on the Rosetta Stone, which is a decree affirming the cult of the then-current Pharaoh of Egypt, Ptolemy V, and names the honours bestowed upon him.

However, the page is not without its flaws.  Namely, it attributes the majority of the credit to Jean Champollion, while claiming that the discovery made by Thomas Young was done by scholars in general.  More significantly, it only mentions that the key Champollion found to deciphering the Rosetta stone was in his 'Lettre à M. Dacier' published in 1822, and does not elaborate on the subject further than that.

As a result, one would have to look to another site or source in order to learn what the key Champollion used was.