In this paper, Ed Codd introduces the relational data model. Codd begins by motivating the importance of data independence: the independence of the way data is queried and the way data is stored. He argues that existing database systems at the time lacked data independence; namely, the ordering of relations, the indexes on relations, and the way the data was accessed was all made explicit when the data was queried. This made it impossible for the database to evolve the way data was stored without breaking existing programs which queried the data. The relational model, on the other hand, allowed for a much greater degree of data independence. After Codd introduces the relational model, he provides an algorithm to convert a relation (which may contain other relations) into first normal form (i.e. relations cannot contain other relations). He then describes basic relational operators, data redundancy, and methods to check for database consistency. Commentary 1. Codd's advocacy for data independence and a declarative query language have stood the test of time. I particularly enjoy one excerpt from the paper where Codd says, "The universality of the data sublanguage lies in its descriptive ability (not its computing ability)". 2. Database systems at the time generally had two types of data: collections and links between those collections. The relational model represented both as relations. Today, this seems rather mundane, but I can imagine this being counterintuitive at the time. This is also yet another example of a unifying interface which is demonstrated in both the Unix and System R papers.