Staging refers to a system of classification that helps doctors talk about the spread of a cancer. Although ocular lymphoma does not have a rigid staging system, an extensive system does exist for melanoma originating in the eye. It uses three main symbols: T, the size of the tumor; N, whether the melanoma has spread to lymph nodes; and M, which indicates if the cancer has spread to other organs. A general system of stages - I, II, III, etc. - is organized, by severity, according to combinations of these three sub-classifications.