Rosetta Stone

English Wikipedia has articles on:

Rosetta Stone and Rosetta Stone (disambiguation)

Wikipedia Wikipedia

the Rosetta Stone

1. A large inscribed stone found near Rosetta, Egypt, in 1799, later used as a basis for understanding many previously undecipherable examples of hieroglyphic writing.
2. (by extension) Any source of information crucial to interpreting.

   My friend’s carefully-compiled notebook was the Rosetta Stone that opened up understanding of the physics lectures.