The complement of a nucleobase is the base to which it bonds in DNA.
The complement of adenine is thymine, and the complement of cytosine is guanine;
see the figure below.
RNA also experiences complementarity in its base pairing bonds during RNA folding,
which occur only in certain locations along the RNA strand. Cytosine still bonds with guanine,
while adenine bonds with thymine's replacement uracil.