Persistent items includes elements that exist over time. For example, 'persons' are a class of the type persistent item. Other examples are 'geographical places', but also more abstract classes such as 'groups'. Persistent items are described through temporal entities, which refer to them.The classes of temporal entities include events located in time which characterize the persistent itmes. Example of temporal entities are events such as a 'birth', 'death' or a 'coronnation'. Each of them has taken place at a specific moment in time.