Which Egyptian script bridged hieroglyphs and the Rosetta Stone's Greek?

Demotic, the everyday cursive script of late Egypt - the middle band of the Rosetta Stone, between the hieroglyphs at the top and the Greek at the bottom. The stone carries one decree written three times, which is what made it a key.

The decipherment was slower and nastier than the story suggests. Thomas Young worked out that the cartouches held royal names spelled phonetically and got a fair way with demotic; Champollion, who had taught himself Coptic - the last descendant of the Egyptian language, still used in church liturgy - saw that the writing was a mixture of sound signs and meaning signs rather than one or the other, and cracked it in 1822. The two accused each other of stealing credit for the rest of their lives.

The key insight was that hieroglyphs are not picture-writing. Europeans had spent centuries assuming each sign was a mystical symbol of an idea, following a late Greek author who said so, and that assumption made the script unreadable for about 1,400 years. Once you accept that a picture of a mouth can simply mean the sound r, everything opens - and roughly three thousand years of Egyptian records became legible again within a generation.