RESIDUES in B-sheets occur in two distinct tertiary contexts : central strands, bordered on both sides by other beta-strands, and edge strands, bordered on only a single side by another beta-strand(1). The Delta Delta G values for beta-sheet formation measured at an edge beta-strand of the IgG-binding domain of protein G(GB1) are suite different from those obtained previously(2,3) at a central position in the same protein. In particular, there is no correlation at the edge position with statistically determined beta-sheet-forming preferences(4). The differences between beta-sheet propensities measured at central and edge beta-strands, Delta Delta Delta G values, correlate with the values of water/octanol transfer free energies(5) and side-chain non-polar surface area for the amino acids(6). These results strongly suggest that, unlike alpha-helix formation, beta-sheet formation is determined in large part by tertiary context, even at solvent-accessible sites, and not by intrinsic secondary structure preferences.