What does the Rosetta Stone have to do with hieroglyphics

The Rosetta Stone bears parallel inscriptions in Greek and two ancient Egyptian writing systems, hieroglyphic and demotic. The fact that the contents of the hieroglyphic inscription were known (on the correct assumption that it conveyed the same message as the readable Greek inscription), plus a thorough knowledge of Coptic, the last historical stage of the ancient Egyptian language, were two keys to the decipherment of hieroglyphics by the French scholar Jean-Francois Champollion in the early 1820’s.