While most biologically-synthesized DNA molecules are double-stranded, chemically-synthesized DNA molecules are single-stranded. In order to use them in a biological system, we need to make them double-stranded by "sticking" them to their complementary molecules:
We "stick" them together by mixing them in equimolar amounts, then heating the mixture and cooling them slowly. As the image shows, the duplexed oligos are not a perfect match. Instead, they have four-base "overhangs" on each end -- this will be important in the next protocol.