system
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human
ELI5: After discovering the Rosetta Stone, how is it that Egyptologists could understand so much of the hieroglyphs based on the limited vocabulary and time period of the Rosetta Stone?

From what I know about hieroglyphs, apparently Egyptologists can translate and understand hieroglyphs and the different meanings they have over the centuries that these hieroglyphs later adopted. All hieroglyphs are understood since the discovery of the Rosetta Stone by the French during the advent of Orientalism in Europe. Apparently, even regional differences in pronunciations and meanings of the hieroglyphs are known as well. How is it that Egyptologists could understand so much of the hieroglyphs based on knowing the meaning of a single document fixed in a specific time period?
gpt
The Rosetta Stone is a piece of a large stele which has the same text in Ancient Greek, Demotic, and Hieroglyphs. Once it was discovered that the Demotic and Ancient Greek texts were effectively the same, work began on translating the Hieroglyphs. The initial translation took twenty years, and it took many more for a more-or-less complete lexicon to be developed.

Demotic script was based on Hieratic, which was based on Hieroglyphs, which made work easier. Demotic and Ancient Greek are phonetic, allowing for the first time for Egyptian names to be read as they were pronounced. Hieroglyphs are both phonetic and logographic (characters represent concepts), which provided researchers with clues as to where they were in the text at certain points. This was also how the Ancient Egyptians were able to represent both foreign names and regional differences in their dialects.

The translation of the Rosetta Stone was only the first step. The translations had to be cross-referenced with known Hieroglyphic texts to test and refine them. More glyphs were added to the known lexicon as this work continued. Other trilingual stele were discovered, providing additional material for checking translations.