Lettre à M. Dacier (full title: Lettre à M. Dacier relative à l'alphabet des hiéroglyphes phonétiques) is the founding text upon which Ancient Egyptian hieroglyphics were first systematically deciphered by Jean-Francois Champollion, largely on the basis of the multilingual Rosetta Stone.
While visiting his brother Jacques-Joseph on September 14, 1822, Jean-Francois Champollion made a crucial breakthrough in understanding the phonetic nature of hieroglyphics, and proclaimed "Je tiens l'affaire!" ("I've found it!") and then fainted dead away.[1]
On 27 September 1822, he exhibited at the Académie des Inscriptions et Belles-Lettres a draft containing eight pages of text to a packed room.
The full and final version was published in late October 1822 by Firmin-Didot in a booklet of 44 pages with 4 illustrated plates.[2]