After graduating from Oxford University and serving in World War II, Codd joined IBM in 1948 as a mathematical programmer; he received a doctorate of computer science from the University of Michigan in 1967. He subsequently developed the principles of the relational model of database management, which were published internally within IBM in 1969 and publicly in 1970 in the seminal paper ‘A Relational Model of Data for Large Shared Data Banks’. Codd continued to develop his theories, but he was not included in the IBM project to implement his ideas. As relational database systems grew in popularity in the early 1980s, he became increasingly frustrated that his vision was not being implemented correctly. In 1985 he published Codd’s 12 rules that defined his criteria for the essential qualities for a database management system to be considered relational.