E.F. Codd’s 12 rules (well, thirteen):
Twelve rules are cited below as part of a test to determine whether a product that is claimed to be fully relational is actually so. Use of the term “fully relational” in this report is slightly more stringent than in my Turing paper (written in 1981). This is partly because vendors in their ads and manuals have translated the term “minimally relational” to “fully relational” and partly because in this report, we are dealing with relational DBMS and not relational systems in general, which would include mere query-reporting systems.
However, the 12 rules tend to explain why full support of the relational model is in the users’ interest. No new requirements are added to the relational model. A grading scheme is later defined and used to measure the degree of fidelity to the relational model.
This particular article seems less important thirty years later, but it was vital in the early days of relational systems to understanding what, precisely, a relational database management system ought to do and—just as importantly—what it ought not do. It wasn’t enough to slap SQL on top of a hierarchical database platform and call it relational.