For the analysis of correlations between the structure and sequence of proteins, we introduced a classification of simplices based on the relative positions of vertex residues in the primary sequence. Two residues were defined as distant if they were separated by one or more residues in the protein primary sequence. Simplices were divided into five nonredundant classes: class {4}, where all four residues in the simplex are consecutive in the protein primary sequence; class {3,1}, where three residues are consecutive and the fourth is a distant one; class {2,2}, where two pairs of consecutive residues are separated in the sequence; class {2,1,1}, where two residues are consecutive, and the other two are distant both from the first two and from each other; and class {1,1,1,1} where all four residues are distant from each other (Figure 2b). All five classes usually occur in any particular protein.