DNA-binding domains (DBDs) frequently have N- or C-terminal tails, rich in lysine and/or arginine and disordered in free solution, that bind the DNA separately from and in the opposite groove to the folded domain. Is their role simply to increase affinity for DNA or do they have a role in specificity, that is, sequence recognition? One approach to answering this question is to analyze the contribution of such tails to the overall energetics of binding. It turns out that, despite similarities of amino acid sequence, three distinct categories of DBD extension exist: (i) those that are purely electrostatic and lack specificity, (ii) those that are largelynon-electrostatic with a high contribution to specificity and (iii) those of mixed character that show sequence preference. Because in all cases the tails also increase the affinity for target DNA, they represent a crucial component of the machinery for selective gene activation or repression.