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.
Nitrogenous bases, phosphate and pentose sugar. Nitrogenous bases, phosphate and pentose sugar are building blocks of DNA. The nitrogenous bases are purines and pyramidines. The purines are adenine guanine, while the pyramidines are cytosine and thymine. The pentose sugar, phosphate and one nitrogenous base form a long chain called polynucleotide chain.