Which ancient writing system was deciphered using the Rosetta Stone?

Egyptian hieroglyphs were deciphered with the help of the Rosetta Stone.

The stone, discovered near Rashid in Egypt in 1799, carries the same decree in three scripts: Egyptian hieroglyphs, Demotic Egyptian and Ancient Greek. Because scholars could read Greek, the parallel text provided a key to understanding the Egyptian scripts.

French scholar Jean-François Champollion announced a major breakthrough in decipherment in 1822. He demonstrated that hieroglyphs could represent sounds as well as ideas and that the script recorded the Egyptian language.

The decree was issued in 196 BC during the reign of Ptolemy V. The Rosetta Stone is made of granodiorite and is now held by the British Museum in London. It did not contain every hieroglyph or explain the entire writing system by itself; its value came from the matching multilingual text.