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.
Hydrogen bonds join the bases of complementary DNA strands. Three hydrogen bonds form between adenine and guanine, while two hydrogen bonds form between cytosine and thymine. Hydrogen bonds join the bases of complementary DNA strands. Three hydrogen bonds form between adenine and guanine, while two hydrogen bonds form between cytosine and thymine.