Fig. 2. Most active sites on Pt single-crystal surfaces for the ORR. *OH sits at the active sites, which are located in all cases at the bottom of the steps ( is provided in each case, see Section S2 in the ESI† for details of its assessment). The Miller indices of the surfaces are provided together with the length of their (111) terraces and step types, namely (111)-like (left) or (100)-like (right). In all cases, the step edges are covered with *O.