 The types 4 through 6 indicate increasing thermal alteration; Type 3 is assumed to be unaltered.Van Schmus, Wood 1967; Sears, Dodd 1988; Brearley, Jones 1998; Weisberg 2006

The modern groups 'V' and 'O' were named by Van Schmus in 1969 as divisions of Type 3, as 'subclass C3V' and 'C3O'.Wasson then added C2M in 1974; since then, C2Ms have generally been shortened to simply 'CM', as have the other groups.