The categorisation of the past into discrete, quantified named blocks of time is called periodization. This is a list of such named time periods as defined in various fields of study. These can be divided broadly into prehistorical periods and historical periods (when written records began to be kept).(Further information: List of archaeological periods) In archaeology and anthropology, prehistory is subdivided around the three-age system, this list includes the use of the three-age system as well as a number of various designation used in reference to sub-ages within the traditional three.