In chemistry, a contact number (CN) is a simple solvent exposure measure that measures residue burial in proteins.^[1] The definition of CN varies between authors, but is generally defined as the number of either C${\displaystyle \beta }$ or C${\displaystyle \alpha }$ atoms within a sphere around the C${\displaystyle \beta }$ or C${\displaystyle \alpha }$ atom of the residue. The radius of the sphere is typically chosen to be between 8 and 14Å.