what does it mean that the two strands of dna are complementary

The two strands of DNA are complementary because they are held together by hydrogen bonds between specific pairs of bases. Adenine always pairs with thymine, while cytosine always pairs with guanine. This base pairing principle means that the sequence of bases in one strand automatically determines the sequence of bases in the other strand. Because of the complementarity of the two strands, having one strand means that it is possible to recreate the other strand. During DNA replication, each of the two strands that make up the double helix serves as a template from which new strands are copied. The new strand will be complementary to the parental or “old” strand.