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A tuple contains exactly one element from the attribute ___domain per attribute, but if the ___domain contains relations, this value would be a relation which in turn may contain multiple tuples and attributes.
A ___domain which contains relations is called a [[finitary relation|nonsimple ___domain]], while a ___domain which does not contain relations is called a simple ___domain. The term ''repeating group'' is analogous to a nonsimple ___domain.<ref>Codd, E.F (1970). "A Relational Model of Data for Large Shared Data Banks". Communications of the ACM. Classics. 13 (6): 377–87. p. 380</ref> Normalization to 1NF is thus a process of eliminating nonsimple domains from all relations.
E. F. Codd uses the terms ''atomic'' and ''nondecomposable'' for elements of simple domains<ref>Codd, E.F (1970). "A Relational Model of Data for Large Shared Data Banks". Communications of the ACM. Classics. 13 (6): 377–87. p. 380</ref>. Thus an atomic value is any value which is not a relation. Such values cannot be decomposed using just [[relational algebra]] operators like selection or projection.
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