E. F. Codd mentioned nulls as a method of representing missing data in the relational model in a 1975 paper in the "FDT Bulletin of ACM-SIGMOD". Codd's paper that is most commonly cited in relation with the semantics of Null (as adopted in SQL) is his 1979 paper in the "ACM Transactions on Database Systems", in which he also introduced his Relational Model/Tasmania, although much of the other proposals from the latter paper have remained obscure. Section 2.3 of his 1979 paper details the semantics of Null propagation in arithmetic operations as well as comparisons employing a ternary (three-valued) logic when comparing to nulls; it also details the treatment of Nulls on other set operations (the latter issue still controversial today). In database theory circles, the original proposal of Codd (1975, 1979) is now referred to as "Codd tables". Codd later reinforced his requirement that all RDBMSs support Null to indicate missing data in a 1985 two-part article published in "ComputerWorld" magazine.