Affymetrix microarrays use base pairing — a process known as hybridization — to help researchers identify unknown DNA sequences that might be present in a sample. Because C always pairs with G, and T always pairs with A, if you know that one side of the chain is ATCATG, you don't even need to see the other side — you know it is TAGTAC. Of course, if that second strand is RNA, the sequence will be UAGUAC.