The term hapticity is use to describe how a group of contiguous atoms of a ligand are coordinated to a central atom. Hapticity of a ligand is indicated by the Greek character 'eta', η. A superscripted number following the η denotes the number of contiguous atoms of the ligand that are bound to the metal. In general the η-notation is only used when there is more than one atom coordinated (otherwise the κ-notation is used, see also hapticity vs. denticity). Hapticity of a ligand can change to accommodate the 18-electron rule.