The crystal structure of a complex of human TATA-binding protein with TATA-sequence DNA has been solved, complementing earlier TBP/DNA analyses from Saccharomyces cerevisiae and Arabidopsis thaliana. Special insight into TATA box specificity is provided by considering the TBP/DNA complex, not as a protein molecule with bound DNA, but as a DNA duplex with a particularly large minor groove ligand. This point of view provides explanations for: (1) why T.A base-pairs are required rather than C.G; (2) why an alternation of T and A bases is needed; (3) how TBP recognizes the upstream and downstream ends of the TATA box in order to bind properly; and (4) why the second half of the TATA box can be more variable than the first.