A measure of the location of an amino acid residue in a protein molecule; the number of α-carbons, excluding those of the adjacent residues in the linear sequence, within an 8 Ã radius of the α-carbon of any given residue; this varies from 0 for residues on a protein's surface to about 15 for those in the interior. Because there is a higher proportion of internal to external residues in larger globular proteins, the values can be normalised by calculation of a relative contact number, i.e. the difference between the average contact number for all residues of a protein and that of any given residue, which varies between -5 and +5.