Ralph introduced the concept of "slowly changing dimension (SCD)" attributes in 1996.
Slowly Changing Dimensions (SCD) - dimensions that evolve slowly over time, rather than changing the time base in the regular calendar. In the data warehouse, it is necessary to follow the changes of the attributes in the dimension in order to make the data historized. In other words, the implementation of an SCD type must allow users to assign the dimension attribute value appropriate for a given date. Example of these dimensions could be: customer, geography, employee.