Ted Codd, whose theoretical work on relational databases stimulated today’s plethora of relational products, defined a fundamental “Rule Zero” for classifying relational database management systems (RDBMSs). The intent of this rule was to help enterprises focus on the requirement for a consistent integrity layer in the RDBMSs they evaluated: “For any system that is advertised as, or claimed to be, a relational database management system, that system must be able to manage databases entirely through its relational capabilities, no matter what additional capabilities the system may support.”