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Why are bilingual inscriptions especially useful for deciphering ancient languages?

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Neo-Assyrian clay tablet from the 7th century BC inscribed with dense cuneiform writing, known as the Flood Tablet from the Epic of Gilgamesh, displayed in the British Museum. — epigraphy The Flood Tablet, 11th cuneiform tablet in a series relating the Gilgamesh epic, from Nineveh, 7th century bce; in the British Museum, London.

Bilingual or multilingual inscriptions are essential for deciphering ancient languages because they provide a bridge between an unknown script or language and one that is already understood. When a text is presented in multiple versions, scholars can use the known language as a key to interpret the corresponding signs, grammar, and vocabulary of the unknown language.12

The most famous example is the Rosetta Stone, which featured the same decree in hieroglyphics, demotic script, and Greek. By comparing the Egyptian texts to the known Greek version, researchers like Thomas Young and Jean-François Champollion were able to identify royal names and eventually establish the phonetic and ideographic values of the hieroglyphs.2

Similarly, trilingual inscriptions from the Achaemenid Empire—often written in Akkadian, Elamite, and Old Persian—have served as vital tools for historians and linguists to unlock the meanings of ancient texts. Without such bilingual keys, many ancient scripts remain virtually undeciphered.31