E.F. Codd's 1970 paper "A Relational Model of Data for Large Shared Data Banks" is one of the most consequential technical documents ever written.
The core idea: represent data as relations (tables), not as hierarchies or networks. Operations on relations always produce relations. The interface is declarative — describe what you want, not how to get it.
Fifty years later, SQL is still the dominant query language. The model was right.