The two strands of DNA are said to 'complement each other.'
DNA contains complementary base pairs in which adenine is always linked by two hydrogen bonds to thymine (A−T). Guanine is always linked by three hydrogen bonds to cytosine (G−C).
In DNA, adenine always bonds with thymine, and cytosine bonds with guanine.
A A C C C T G A G T C T - (one DNA strand is given)
T T G G G A C T C A G A - (complementary sequence of bases in the other strand of DNA)