True and false quadruplets generated from DSSP-defined β-strands. 6a, 6b: Residues i, j and i+1, j-1 shown paired with green broken lines form the true quadruplet. For every such quadruplet four false quadruplets (shown with red and blue broken lines) are possible: j, i+1, i+2, j-1 and j-1, i, i+1, j-2 in fig. a; i, j-1, j, i-1 and i, i+1, j, j+1 in fig. b. These quadruplets were scored to find the difference in scores between true and false quadruplets. True and false scores for quadruplets generated from DSSP output for parallel β-strand coordinates (fig. c) and antiparallel β-strand coordinates (fig. d). Cutoff c1 (45) and c2 (46) in fig. c are the scores of the best false quadruplet and the worst correct quadruplet respectively for parallel β-strand data. Cutoff c1 (39) and c2 (44) are the scores of the best false quadruplet and worst correct quadruplet respectively from antiparallel β-strand data. Cutoffs c1 and c2 are used as cutoffs to differentiate between grade 1 and grade 2 quadruplets in our algorithm.