Used_in The definition of C++ [ c++.syntax.html ]. Used_in The definition of Java [ java.syntax.html ] Notation This uses my XBNF Extended BNF Notation where | indicates or, (...) indicates priority. For more information see [ intro_ebnf.html ].
To look at it in another way: Thymine(uracil in RNA) and cytosine are pyrimidines. Pyrimidines are single-ringed organic base that will only attach to purines, or adenine and guanine by hydrogen bond. Because cytosine and thymine are both pyrimidines, it cannot form a hydrogen bond with each other.
It is always paired with adenine by two hydrogen bonds. Thymine is a pyrimidine that is found only in DNA molecules and is synthesized from uracil. Uracil is a pyrimidine type nitrogenous base that is found only in RNA molecules. It always pairs with adenine. Chemical difference of uracil and thymine is very small. Uracil has a hydrogen atom at C-5 carbon while thymine has a methyl group at the same carbon.